Uncertainty is not our friend

I just finished the SDGAcademyX: CCSI001 course Climate Change: The Science and Global Impact.

The course covers the basic principles of atmospheric science, methods of climate data collection, and tracking of greenhouse gas emissions. It introduces basic climate modeling and explores the impact of various greenhouse gas emissions scenarios. Finally, it outlines the impacts of climate change on environmental, social, economic and human systems, from coral reefs and sea level rise to urban infrastructure. The course follows the general outline of the 5th Assessment Report of the United Nations Intergovernmental Panel on Climate Change.

Our course lecturer was Michael Mann, Distinguished Professor of Atmospheric Science at Penn State University.

I feel the need to share some of the course material, for anyone who is interested and has no time to enrol in the course. In particular I feel the need to share the course content which covers Climate impacts on human beings and human civilisation, but also other systems, other living things, our environment, tipping points and the final module which covers ‘What Is Our Path Forward?’ and ‘There is hope’.

Underneath are copies transcripts of some of the video lectures of modules 7 and 8. The video’s include much more information than the transcripts alone, such as footage of events and graphs. Each module comes with a list of links to further readings. I do therefor recommend you to enrol in this course. By the end of this course, you will: Develop a deep scientific understanding of HOW the climate system has been changing; Articulate WHY the climate system is changing; Understand the nature of these changes; Develop a systems thinking approach to analysing the impacts of climate change on both natural and human systems.

Complete copy of video transcripts of modules 7 and 8 – shared with permission (Creative Commons Attribution Non Commercial Share Alike 4.0 International) :

Module theme 7: Climate Change Impacts: The Future for People and Planet

Chapter 7.1 examines the different carbon feedback cycles in the Earth's climate system.

So this next module is about climate impacts, and that can include impacts on human beings and human civilisation, but also other systems, other living things, our environment. We talked in an earlier module about the concept of carbon cycle feedbacks. The airborne fraction of CO2 in the atmosphere has increased by only half as much as it should have given the emissions that we have added through fossil fuel burning and deforestation and other human activities. We know that CO2 must be going somewhere, namely being absorbed into other subsystems of the climate: the ocean and the soil and vegetation in particular. Indeed carbon is being absorbed by various reservoirs that exist within the global carbon cycle. Only 55 percent of the emitted carbon has shown up in the atmosphere. While roughly 30 to 35 percent appears to be going into the oceans, and another 15 to 20 percent into the terrestrial biosphere. Now on the one hand that might seem like good news that some of that carbon isn't building up in the atmosphere, it's being absorbed by these subsystems, but the problem is that this pattern of behaviour may not continue. There's no guarantee that the ocean and terrestrial biosphere will continue to be able to absorb the same fraction of carbon emissions as time goes on, and that leads us into a discussion of so-called carbon cycle feedbacks.

If we consider the oceans, there are a number of factors that could lead to decreased uptake of carbon as time goes on. Like a warm can of coke which loses its carbonation when you warm it up and remove the top, the ocean CO2 solubility decreases as the ocean warms. When we look at carbon uptake in the ocean we see that one of the primary regions of uptake is the North Atlantic. This is in part due to the formation of carbon burying deep water in the region. In a scenario we explored earlier, the conveyor belt ocean circulation is likely weakening. This could eliminate one of the oceans key carbon burying mechanisms and allow CO2 to accumulate faster in the atmosphere. On the other hand the biological productivity of the upwelling zone, of that code tongue region in the eastern and central equatorial Pacific, is a net source of carbon to the atmosphere from the oceans. More El Nino like conditions in the future could suppress this source of carbon, but more La Nina light conditions could increase this source, further accelerating the build-up of CO2 in the atmosphere. Unbalanced it is likely that this uptake will decrease over time, yielding a positive net carbon cycle feedback.

Now as CO2 builds up in the atmosphere and some of that CO2 diffuses into the ocean, it's literally acidifying the ocean. This is sometimes called ocean acidification and it's sometimes considered climate changes evil twin because it is another negative consequence on our environment from the burning of fossil fuels and the accumulation of CO2 in the atmosphere. Now some of these carbon cycle feedbacks relate to the phenomenon of ocean acidification, and to be specific about what ocean acidification is, that increasing CO2 in the atmosphere leads to more dissolved CO2 in the ocean, and that takes the form of the bicarbonate ion in the ocean. Other ocean carbon cycle feedbacks relate to this phenomenon of ocean acidification, which is associated specifically with the increasing concentration of the bicarbonate ion in the ocean as atmospheric CO2 diffuses into the ocean.

On the one hand, this process interferes with the productivity of calcite skeleton forming ocean organisms such as zooplankton which bury their calcium carbonate skeletons on the seafloor when they die. This so called oceanic carbon pump is a key mechanism by which the ocean buries carbon absorbed from the atmosphere on long timescales. So any decrease in the effectiveness of the oceans carbon pump would represent a positive carbon cycle feedback, increased accumulation of CO2 in the atmosphere. On the other hand, since calcifying organisms release CO2 into the as they build their carbonate skeletons, a decrease in calcite production by these organisms will reduce CO2 [inaudible] to a negative carbon cycle feedback. So clearly we are once again dealing with uncertainty, there's some uncertainty in the extent to which some of these positive and negative carbon cycle feedbacks may be realised, but historically we've seen that uncertainty is not our friend.

As we've learned more about the system we've often found that it is more sensitive to our burning of fossil fuels, and that the changes may be larger than the science suggested just years ago. There are a number of other carbon cycle feedbacks that apply to the terrestrial biosphere, they vary anywhere from a strong negative to a strong positive feedback. Among them are A. warmer land increasing microbial activity and soils which releases CO2 into the atmosphere, a small positive feedback. B. increased plant productivity due to higher CO2 levels, a potentially strong negative feedback.

Finally there is the negative silicon rock weathering feedback which we know to be a very important regulator of atmospheric CO2 levels on very long geological timescales. In short, a warmer climate with its more vigorous hydrological cycle leads to increased physical and chemical weathering, that's the process of taking CO2 out of the atmosphere by reacting it with rocks. Now that takes place through the formation of carbonic acid which dissolves silicate rocks producing dissolved salts that runoff through river systems eventually reaching the oceans and getting deposited on the ocean floor. While each of these potential carbon cycle feedbacks are uncertain in magnitude and even in sign in some cases, and see the various coloured bars in the figure. The net result of all these feedbacks appears to be a net positive carbon cycle feedback, that is to say on the whole this is likely to add to further accumulation of CO2 in the atmosphere and additional warming.

There are other potential positive carbon cycle feedbacks that are even more uncertain but could be quite sizable in magnitude. Among them are methane feedbacks related to the possible release of froze methane currently trapped in thawing Arctic permafrost and so-called clathrate, a crystalline form of methane that has found in abundance along the continental shelves of the oceans. That could be destabilised by Modest ocean warming. Since methane is a very potent greenhouse gas, such releases of potentially large amounts of methane into the atmosphere could further amplify greenhouse warming and the associated climate changes. The key potential implication of a net positive carbon cycle feedback is that current projections of future warming may actually underestimate the degree of warming expected from a particular carbon emissions pathway. Again we come back to the threats inherent in uncertainty.

Chapter 7.2 explains the impact on that different levels of sea level rise would have on people and societies around the world.

Let's start to talk now about some of the human impacts of climate change. And there's none perhaps that is more profound than the impact of sea level rise. Now sea level is projected to rise potentially as much as two meters, six to eight feet over the next century, and perhaps as much as five meters, more than 20 feet by 2300, given business-as-usual burning of fossil fuels. Scenarios such as ten meters of sea level rise are not out of reach should, for example, the West Antarctic Ice Sheet collapse more abruptly than is indicated by uncertain current model estimates. The impacts of rising sea level will be felt differently by different nations and regions. For low-lying island nations like the Maldives or Bangladesh or the Low Countries of Europe, even the lower-end sea level rise scenarios represent a distinct threat. In fact, some island nations such as Tuvalu and Kiribati and parts of Alaska have already made contingency plans for evacuation. Even moderate sea level rise much less than a meter can lead to significant increases in coastal erosion and other problems such as salt water intrusion of freshwater supplies, where saline water penetrates increasingly inland through estuaries and tributaries contaminating freshwater ecosystems and aquifers relied upon for freshwater supply. We see this happening regularly now in places like Miami.

Well, even moderate sea level rise would be problematic. On the other hand much larger 1 to 5 meter sea level rise is projected over the next one in two centuries under business-as-usual emissions. With one meter of sea level rise we would see the disappearance of low-lying regions, the Gulf Coast including the Florida Keys. At five to six meters of sea level rise we would see the loss of the southern third of Florida and many of the major cities of the Gulf Coast and east coast of the US. At 10 meters of sea level rise, New York City would be submerged. Many of the other largest coastal cities in the world like Rio de Janeiro, Shanghai, Sydney would be inundated as well.

There's several different ways we can measure the impacts of sea level rise. One can measure the cost of increasing levels of sea level rise in terms of, for example, A. the loss of land area or B. the damage to our economy is measured by gross domestic product, GDP, or C. the increase in population impacted either directly by inundation or increased coastal erosion, or indirectly for example by saltwater intrusion into freshwater supply. In the scenario of 10 meters of sea level rise, not entirely out of the question on a time scale of a few centuries, the global costs as measured by any of the above metrics are rather staggering. More than 5000 square kilometres of coastal land lost, nearly 3 trillion dollars of GDP lost, and more than a third of a billion people exposed to direct or indirect impacts of sea level rise. The cost of civilisation of those sorts of impacts is almost unimaginable.

The good news is that we can still avert such a catastrophic future by bringing our carbon emissions well below the business-as-usual trajectory, by stabilising them below levels that will lead to more than 2 degrees warming of the planet. These sorts of sea level rise impacts convey the catastrophic impact that business-as-usual burning of carbon would have, and it drives home the urgency of acting to avert a catastrophic future. The good news is that there is still time to bring our carbon emissions down well below the business-as-usual scenario, enough that we keep warming of the planet below 2 degrees Celsius relative to the pre-industrial, and we will avoid these high-end sea level rise estimates.'

Chapter 7.3 looks at the extinction of different species and ecosystems as global warming continues.

Climate change is already having a demonstrable impact on natural ecosystems, and this is particularly evident when looking at niche, for example mountain and high latitude environments where species are highly adapted to the prevailing past climatic conditions, and have gone extinct or in the process of potentially going extinct because of rapidly shifting climate conditions.

The poster child, perhaps, of climate change-related extinction is the golden toad. This magnificent amphibian once ranged throughout the high elevation of cloud forests of Monteverde, Costa Rica. First discovered in the 1960s, the toad appears to have gone extinct in the late 1980s. Scientist Allan pounds and his colleagues have argued that the demise was due to climate change associated with a very long term drying as the cloud forests have been lifted to higher and higher elevations by a warming atmosphere. Other scientists have since noted that the influence of climate change in this extinction event was likely somewhat more subtle with the immediate factor having been the outbreaks of a fungus known as Chytrid. The drying conditions may have made the golden toad more susceptible to these fungus outbreaks. So climate change related extinctions aren't just theoretical, we are already seeing them happen. Another poster child is the polar bear. Polar bears require a sea ice environment to hunt their primary food source, seals. It is very possible that with a little more than 1 degrees Celsius warming, that environment will essentially disappear within the next century. That is to say, there will be an increasingly long ice-free period from the spring through the fall over most of the polar bears range. This means that the season during which polar bears can hunt for their primary food source is getting shorter and shorter.

And then of course there are the world's coral reefs. While they occupy less than 0.1% of the world oceans, coral reefs are home to 25% of all marine species, constituting a major reserve of marine biodiversity. Coral reefs are threatened by the twin impacts of fossil fuel-burning warming ocean waters leading to more widespread bleaching of coral reefs and ocean acidification which is literally dissolving the coral reefs making it more difficult for corals to form their skeletons. Coral bleaching, ocean acidification is combining with other threats to coral reefs, pollution, ultraviolet radiation from ozone depletion in sort of a perfect storm of consequences leading scientists to conclude that if we continue with business as usual, most of the world's coral reefs will be gone within a matter of decades. Indeed we have seen the disappearance of many of the coral reefs in the Caribbean, and with the latest bleaching event a multi-year bleaching event of the Great Barrier Reef off the coast of Australia, ninety percent of the Great Barrier Reef, the world's largest coral reef suffered from bleaching and is at threat of mortality if there isn't a recovery.

These are just a few illustrative examples of a much broader threat that climate change poses to animal species. It's convenient to summarise the impacts on animal species and ecosystems and biodiversity in terms of a thermometer scale that characterises the degree of species loss as a function of additional warming. We already saw that amphibians in particular are under threat from global warming with less than two degrees Celsius additional warming, we might see widespread disappearance of amphibians, and above 2 degrees Celsius warming a loss of as much as a third of all species. At three degrees additional warming, we could see as much as a 50% loss of all species worldwide. At four degrees Celsius warming, that rises to as much as 70%. This has led some scientists to state that we are now causing the sixth major extinction event in geological history.

Chapter 7.4 examines the effect of too much water and too little water as a result of climate change.

Water is essential to life and it is essential to human civilisation. Either too much or too little water is a problem. Climate change may ironically give us both. The greatest threat is the uncertainty of increasingly irregular. and shifting patterns of precipitation. In some regions like the desert southwest of the US climate change threatens to reduce freshwater availability due to both decreased winter rainfall and snowfall that ultimately feeds major reservoirs through spring runoff.

Current projections are that lake Powell which provides southern Nevada with much of its freshwater supply may run dry within a matter of decades extrapolating recent drying trends. These decreases in water supply are on a collision course with demographic trends as population centres, such as Las Vegas and Phoenix continue to expand in size. Similar scenarios are likely to play out in southern Europe, the Mediterranean, the Middle East, southern Africa and parts of Australia. Other regions meanwhile are projected to get too much water. Bangladesh already threatened by rising sea levels is likely to see increased flooding from the intense rainfalls expected with a warmer more moisture laden atmosphere.

Climate change is likely to challenge global food security, but the situation is complicated. Longer growing seasons in northern latitudes could prove favourable for growing crops but at the same time more extreme weather events can damage crops and lead to the interruption of food distribution systems. Even moderate warming is likely to lead to substantial decreases in productivity for key cereal crops grown in the tropics: rice, wheat, sorghum, maize. These crops are growing at what is essentially their optimal temperature, and any warming leads to substantial decreases in yield.

Added to the mix is the direct impact of the increase in ambient CO2 concentrations. There's some empirical evidence that the impact of so-called CO2 fertilisation could also lead to increases in productivity. Plants require CO2 for photosynthesis so to the extent that CO2 is a limiting factor in cereal crop growth, increasing CO2 levels might increase productivity, yet there are additional factors that might mitigate this effect. Large parts of the tropical and subtropical continents are projected to see drying soils as a result of anthropogenic warming.

One exception is Central and Eastern equatorial Africa, but there's little consensus among models on that. To take in CO2 for the purpose of photosynthesis plants must maintain open stomata, but at the same time this increases loss of moisture through evapotranspiration which is a problem as conditions become drier and water itself becomes a limiting factor.

Indeed any increase in drought stress could easily offset the benefits of longer growing seasons in extra tropical regions. The overall evidence when we take climate model simulations and we use them to drive crop models is that in extra tropical regions with potentially longer growing seasons, we could see increased agricultural productivity but we will see substantial decreases in tropical regions.

A similar pattern holds for livestock, since livestock yields depend on food stocks and agricultural productivity.  For warming exceeding 3 degrees Celsius we begin to see sharp decreases in global agricultural yields. Some of the limitations of these projections should be kept in mind, however, they may be overly optimistic in fact because they don't account for other potentially detrimental climate change impacts such as decreased freshwater supply for irrigation, severe weather events such as the catastrophic Pakistan floods and Russian wildfires of 2010, or the unprecedented extreme weather events that played out in Summer 2018 across the entire northern hemisphere which destroyed crops and disrupted food distribution systems. In fact, some of these extreme weather events have been blamed for recent spikes in global food prices.

The impact of climate change on extreme weather events is an important wildcard. It could lead to far sharper losses in agricultural productivity, livestock, and it could further complicate our ability to distribute food around the world.

Chapter 7.5 shows how climate change can affect global health and disease, focusing on the movement of tropical diseases to nontropical zones as the Earth warms.

Climate change is likely to impact human health in a number of ways. On the one hand we might expect decreased mortality from extreme cold, but the flip side is a dramatic increase in warm extremes and heat waves. The young and elderly as well as the poor who are less likely to have access to modern air conditioning, for example, are most at risk. The toll of the unprecedented heat wave in Europe of summer 2003 where more than 30 thousand lives were lost is a possible harbinger of the impact of future more frequent and intense heat waves, and to a lesser extent so are the European heat waves of 2006, 2010, and in North America 2006 and 2010. Moreover, the sorts of extreme weather events we've seen in recent summers, particularly summer 2018, unprecedented heat waves and wildfires and floods have led to the loss of thousands of lives. The unprecedented strike of Hurricane Maria on Puerto Rico in fall 2017 led to thousands of lost lives. Other weather extremes may have human health impacts, in some cases, such as the physical damage and loss of life from landfalling hurricanes this is obvious. But there are many other examples. Intense rainfall events leading to flooding can cause physical harm or create conditions that favour the spread of disease or lead to various ailments.

Drought conditions pose the obvious threat of limiting fresh water supply but they can also favour disease and malnutrition. Once again the impacts fall disproportionately on the poor who are least able to afford clean water, electricity, and modern health care. Climate change is also likely to lead to the spread of various types of infectious disease. Many of these diseases are spread by so-called vectors, pests such as insects and rodents who are capable of spreading the disease far and wide. In many cases the range the vectors is restricted by climate. Diseases such as West Nile fever and malaria, for example, are spread by mosquitoes. Temperate regions with killing frosts are thus relatively inhospitable to the disease as they interrupt the lifecycle of vector and thus the disease itself.

As the planet warms and cold regions retreat poleward, we can expect the regions where disease is currently classified as tropical diseases are endemic to spread well into the extra tropics. The outbreak of West Nile virus in New York State in 2005, for example, was likely due to an unusually warm winter which allowed mosquitoes to persist through much of the year. The warming of the north eastern US in recent years is also led to the expansion of Lyme disease.

Some of my own research involves the impact of climate change on malaria. The problem is complicated in part because it isn't just average temperatures that determines how rapidly the malaria parasite can reproduce, it turns out that there's a threshold dependence on temperature. The malaria parasite reproduces at an exponentially greater rate above a particular threshold temperature, roughly twenty degrees Celsius. This is why Highland tropical African cities such as Nairobi with an elevation of nearly five thousand feet and a mean annual temperature of 19 degrees Celsius are generally free of malaria even while surrounding lowland regions must contend with the disease. This threshold dependence on temperature also implies that one must not only consider the average temperatures but also the variability of temperatures to assess possible impacts on the spread of malaria. As temperatures warm, that twenty degrees Celsius isotherm moves up in elevation and pretty soon Nairobi will not be immune from the impacts of malaria, a very large population of people in Africa will now have to contend with this disease. In summary health impacts of climate change are likely to include increased mortality due to more frequent and intense heat waves, increased spread of disease from more widespread flooding and drought, threats to health and life from more increased storm damage, and the poleward spread of tropical disease with warming temperatures.

Chapter 7.6 looks at how climate change has and will impact matters of national security. It focuses on new coastlines, new shipping routes, refugees, and terrorism all as a result or in part due to climate change.

Given the fierce competition for limited resources, be it food, water, land, etc, it is reasonable to draw the conclusion that climate change may challenge national and international security.

One recurring theme and discussions of national security impacts is the potential military implications of retreating Arctic sea ice. In recent years the mythical Northwest Passage has finally opened up on a semi-regular basis, that is to say it is now possible over part of the year for ships and submarines to travel unimpeded from the Labrador sea through the Arctic Ocean into the Pacific Ocean. As the trajectory of sea ice retreat continues and the open channels widen and deepen, it will likely be possible for large military vessels, ships, and submarines to make this route. That would have deep implications for national security. Suddenly the various nations bordering the Arctic Ocean would be competing for and defending a new Arctic coastline against potential invasion and military attack.

Other scenarios involve the principle that increased conflict between nations and cultures may arise from so-called environmental refugee-ism, people fleeing regions no longer fit for habitation for other currently occupied regions, thereby increasing the competition for resources.

As parts of sub-Saharan Africa such as Senegal become too dry and inhospitable for subsistence agriculture, for example, there has been a flood of human refugees fleeing the environment to the less arid South, like Ghana. Another scenario is that the extremely large populations of interior Nigeria driven by drying conditions flee for the mega city of Lagos to the south where there is heightened competition for food and water resources. Adding to the incendiary mix is the skirmishes that might break out among groups and individuals fighting over the last remains of the disappearing oil reserves of the Niger River Delta and the cronyism and political corruption that may ensue.

Consider also the impact of an increasingly dry Middle East. Some have argued that it is the competition for scarce water resources over the years that has driven much of the Middle East conflict including the Arab Spring events of 2010 and be ongoing as of 2018 Syrian crisis. These events also have implications for international terrorism. Syria for example is experiencing the worst drought in at least 900 years according to paleo climate evidence, and that drought has forced rural farmers into the cities where they are now competing for food and water and space with the people who already inhabit the cities. That leads to increased competition and conflict. And that environment of conflict has been used as a very effective recruiting tool for terrorist organisations like ISIS which have exploited the impacts of this drought.

Some have argued that we should. focus less on climate change and more on issues like global terrorism. Well that sort of argument involves a deep fallacy because as we've seen the impact of climate change has direct implications for competition for resources, conflict, and indeed global terrorism. In summary the national security impacts of climate change include the need for additional national defences as new shipping routes and coastlines open as a result of diminished Arctic sea ice and increased. conflict arising from the competition between nations and groups for diminishing land, food, and water resources.

Chapter 7.7 looks at the different planetary tipping points from climate change, such as the permanent melting of the continental ice sheets.

We will wrap up our discussion of the science of climate change with a discussion of so-called tipping points. Tipping points are important because they represent possible threshold responses to climate forcing like increasing greenhouse gas concentrations. While many of the climate change impacts we have looked at, like surface temperature increases are projected to follow increasing atmospheric CO2 concentrations in a relatively smooth continuous manner, there are other responses that can be more abrupt. Only a certain amount of warming takes place and some component of the climate system abruptly transitions to another regime. We saw an example of this sort of behaviour in our discussion earlier in the course about the role of the ice albedo feedback in the long term evolution of Earth's climate and how Earth's climate system can have multiple steady states for a given amount of incoming solar radiation.

Another example related to this is what happens to the continental ice sheets when you start to melt them, the Greenland ice sheet and the Antarctic ice sheet today, and once you start to melt ice those positive feedbacks kick in, you have ice shelves that begin to collapse, you have ice cliffs that begin to calve into the ocean, and pretty soon the process feeds on itself, it accelerates, and it's difficult to stop it. Even if you bring CO2 levels back to pre-industrial levels, you won't stop the melting of the ice sheets and what will ultimately result in tens of meters of global sea level rise.

Another example that we saw was the slowdown of the ocean conveyor belt circulation. Once you melt enough ice and it flows into the North Atlantic and freshens those waters, that ocean circulation pattern collapses potentially and there's no way to bring it back at least on human timescales.

A few potential tipping points in the climate system are related for example to the ENSO phenomenon, the El Ni?o phenomenon. Given the complicated physics that describe El Ni?o, it's possible that even a modest additional amount of warming could fundamentally change the regime of behaviour of ENSO and all of the global impacts that that has. Moreover, systems like the Indian summer monsoon are very sensitive, potentially, to modest changes in climate and we could potentially shut down that critical component of the climate system that provides water supply to millions of people in the world.

Like a snowball running out of control down a hill, these tipping points lead to changes in the climate system that we cannot reverse on human timescales. Sometimes this is framed in terms of a cliff that we walk off after a certain amount of warming. Two degrees warming, you walk off the cliff, all of these tipping points kick in and there's no way to reverse them. In reality we don't know where the different tipping points are, the collapse of the Thermohaline circulation, the commitment to the melting of the entire West Antarctic Ice Sheet, we just know that these tipping points lie out there like mines in a minefield and the only sensible strategy is to not go further out on to that minefield lest we begin to trigger more and more of these irreversible and truly catastrophic changes in the climate system.

Module 8 Theme What Is Our Path Forward?

Chapter 8.1 part 1 looks at specific geoengineering measures as ways of mitigating climate change. Two measures focused on are carbon capture and sequestration and direct air capture.

Ok so now you are equipped with an understanding of the science behind climate change, the next question is: what do we do? The answer lies in mitigation which simply put is trying to do something about the problem. Fundamentally, there are two different forms of mitigation that have been widely discussed. Geoengineering and reduction of carbon emissions.

We'll start with geo-engineering and a quick analysis of its various forms. Geoengineering is the intentional manipulation of our environment at the global scale. Anthropogenic impacts on our climate thus do not qualify as geo-engineering because their intent was not to change our climate. Climate change was an unintended consequence of the burning of fossil fuels. Geoengineering at this point is still largely theoretical in the sense that has not been deployed at the global scale that would be necessary to have a substantial impact on the climate, but there are proofs of concept. There have been demonstrations or efforts to demonstrate that these schemes could potentially be viable.

The first example that we'll talk about is carbon capture and sequestration, or CCS. Now some would argue that this isn't actually even geoengineering, it's a different form of mitigation, but we will treat it as a matter of geoengineering, and arguably one of the safer of the proposed geoengineering schemes, at least the one that is least intrusive with respect to the Earth system.

The main idea behind CCS is to prevent the Carbon released from fossil fuel burning from ever getting into the atmosphere. In principle, this would allow for energy generation from the burning of fossil fuels like coal with zero or at least near zero carbon admissions. Now one of the limitations is that CCS can only be done at the site of fossil fuel burning. It can only be done, for example, at a coal-fired power plant and in fact the geology, and conditions, and many other factors, local factors need to be conducive to the burial of carbon beneath the surface of the earth. So it's unclear that this could be deployed at the global scale that would be necessary for it to make a big difference. CCS is only economical when it can be applied to large point sources. In the context of energy generation, this applies almost exclusively to coal-fired power plants. CCS can also be used, however, to capture and sequester carbon emissions resulting from industrial processes, such as steel and cement manufacturing, or petroleum refining, and paper mills.

Now, there have been a number of examples, proofs of concept, that have demonstrated that this can be done, but currently the cost is prohibitive, and at a time when coal is already struggling to compete with other sources of energy in the market place, it's unclear that coal burning with CCS, with the additional costs that come with it, would possibly be competitive against, for example, newer sources of renewable energy.

Another type of geoengineering is what's known as direct air capture, this involves in essence, putting the genie back in the bottle. Unlike CCS, which targets emissions from major point sources like coal-fired power plants before they escape into the atmosphere, in this case, the CO2 is already made it into the atmosphere, and the objective is to literally take it back out of the atmosphere and bury it. One simple way to do this is to grow more trees, just as deforestation adds CO2 to the atmosphere, reforestation can, in principle, take CO2 out of the atmosphere. Of course, things are not as always, as easy as they seem. When the trees die, much of the carbon makes it back into the atmosphere as the organic material decomposes. Though the carbon burial process is incomplete and inefficient, there are problems with this approach. Growing more trees in extratropical seasonally snow-covered regions have the unintended consequence of actually warming the climate by decreasing the Earth's winter and early spring albedo.

Now there's a scheme that has been proposed by Klaus Lackner of Arizona State University that involves the manufacturing of millions of structures that we might think of as artificial trees. Like trees, they take CO2 out of the atmosphere, but they are much more efficient at doing it, and all of that carbon can be buried beneath the surface. Currently, however, the cost of doing this is prohibitive. There are much less expensive ways of dealing with climate change. Arguably much cheaper than this artificial technology would be actual reforestation. Planting trees, particularly in tropical regions that have been deforested, which would be good for the environment and arguably for ecosystems and our environment. Nonetheless, we might have to turn this technology as a stop-gap measure if other efforts to mitigate climate change have proven unsuccessful. In limiting carbon levels to dangerous levels in the atmosphere, we might have to turn to something like direct air capture as a stop-gap measure to prevent catastrophic warming of the planet.

Chapter 8.2 continues the discussion on specific geoengineering measures, focusing on solar radiation management and iron fertilisation of the oceans.

The next category of geoengineering is what is sometimes called solar radiation management which is really a euphemism for blocking some of the incoming shortwave radiation from the sun so that it doesn't reach and warm the surface. Now one of the more discussed versions of solar radiation management involves mimicking what we've already seen a volcanic eruption can do to the climate. Take for example the 1991 Mount Pinatubo eruption. It put large amounts of sulfate aerosol into the stratosphere which sat there for several years cooling the global climate by about a half a degree Celsius. Now if we could do the equivalent of setting off a Mount Pinatubo eruption roughly every three years, it turns out that that would provide enough negative radiative forcing to largely offset the warming effect of increased greenhouse gas concentrations. So it might sound like a sensible scheme, shooting large amounts of sulfate aerosol up into the stratosphere with cannons and doing it often enough that we essentially have the equivalent of a perpetual volcanic eruption, enough so to offset the direct warming effect of greenhouse gases through the increase in the long-wave radiation that comes down towards the surface.

Now in very simple models this game seems to work quite sensibly, however when scientists actually implement this sort of scheme in a fully coupled three-dimensional global climate model, what we find is that the pattern of the cooling from the aerosols won't look like the mirror image of the warming effect of greenhouse gases, so while in the global average temperatures right, might remain relatively constant, that would be at the expense of some regions warming even faster while other regions cool. Moreover, modeling experiments show that we would tend to cool the continents which would actually slow down the hydrological cycle and lead to less rainfall and precipitation in continental regions worsening drought. We would also likely worsen the acid rain problem and ozone depletion through the chemical influence of these sulphur particles in the stratosphere. This underscores the principle of unintended consequences that accompanies most geoengineering schemes, which is to say, we are toying with a system, the global climate system that we don't understand perfectly, and it's possible that if we implement these schemes, we could end up worse off than we had before. In a sense it violates the equivalent of the medical oath to first do no harm. If we engage in some of these schemes like solar radiation management, we could actually do direct harm to the climate on top of the warming that were causing by burning fossil fuels and increasing the greenhouse effect.

Finally we get to a scheme that's known as iron fertilisation of the oceans. In this case, what's proposed is to put iron nutrients into the ocean and because this is a limiting nutrient for algae in large parts of the global oceans, the North Atlantic Ocean in particular, putting iron in the upper part of the ocean could in essence fertilise the ocean by leading to greater biological activity. The principle here is that in large parts of the world's oceans, biological productivity is limited by the amount of nutrients in the upper ocean and in particular iron, and so if you put iron into the upper ocean you may fertilise the production of algae, photosynthetic algae that take CO2 out of the atmosphere and are eaten by other organisms which ultimately die and fall to the bottom of the ocean depositing that carbon on the ocean bottom. Now in this case, unlike direct air capture we're not working on the shortwave part of the energy budget the sunlight coming down what we're trying to do is reduce the long-wave radiation produced by greenhouse warming by decreasing atmospheric CO2.

Now there have been a number of tests that have been done to see if this mechanism is viable and it turns out that by and large those tests reveal some, some basic problems with this approach. One of which is that when the oceans are fertilised at least on a regional basis, though there is some increase in algae productivity, what happens is that the carbon simply cycles through the upper ocean more rapidly, it doesn't actually get deposited on the ocean floor, and so it's unclear that there would be any net export of carbon out of the atmosphere into the ocean and down to the deep ocean, and it would be expensive to implement at a large scale. And finally there's other evidence that getting back into the principle of unintended consequences, iron fertilisation may selectively favour certain types of harmful algae like those algae that cause red tides, a reminder that these sorts of uncertainties are not our friend, and that there's great peril in implementing massive planetary intervention schemes when we're dealing with the only planet we know that can support us and other life. Once again we see the principle of unintended consequences rearing its head. If we were to implement this scheme we could end up worse off than where we started.

When it comes to planetary scale interventions with a system, the Earth system that we don't understand perfectly, and which provides the only home for life that we know of in the universe, one might argue that caution should be the ruling principle and in this case it would be unwise to implement these sorts of schemes without knowing the full range of consequences. So what does the science tell us about geoengineering as a potential scheme for mitigating climate change. Well as we've learned, the principle of unintended consequences reign supreme. With all of these schemes, there are potential unintended consequences that could have dire implications for us and our environment. Moreover, the notion that a solution to the climate change problem might simply be to engage in additional planetary scale interventions with the earth system can be used as a crutch, an excuse for not engaging in the difficult but essential work of actually bringing down our carbon emissions and getting off of fossil fuels. So in short if you're looking for a solution to the climate change problem, geoengineering probably isn't the right one.

Chapter 8.3 explains why emissions reductions are the path forward for climate change reduction.

So if we agree that geoengineering probably isn't the right approach to solving the climate problem, what is? Well in short it's solving the problem at its source which is stopping the continued burning of fossil fuels and other activities that are generating greenhouse gases and warming the planet. This is a course about the science of climate change and that's been our primary focus. On the other hand, no discussion of climate change should go without some consideration of the solutions. Now there are a number of other sources for those of you who are interested in learning much more about the topic of climate change mitigation, solutions, policy. Nonetheless we will at least try to provide a basic summary of those areas so that our treatment of the topic is as comprehensive as possible. So when we look at the various sources of greenhouse gas emissions, we see that the lion's share roughly, 2/3 of our emissions, come from the burning of fossil fuels for energy and transportation. Now the remaining third is distributed among a number of areas including agriculture and land use, but there is no magic bullet. Carbon is generated from all activities, all sectors of society and we need to focus on each of them if we are going to bring down our carbon emissions to the levels necessary to avoid catastrophic warming. So we've seen that carbon emissions are generated from activities across all sectors of human society. And what that means is that any solution to the climate problem is going to require changes in behaviour across our society.

That involves individual actions but also collective actions. From an individual standpoint there are many things that we can do to reduce our own individual carbon footprints. In many cases these actions are what we call no regrets actions. They are things that make us healthier, they save us money, and they reduce our carbon emissions. Whether it's bicycling to work rather than driving or using public transportation, reducing your travel, changing your dietary patterns to make them more carbon friendly. There are so many things that we can do in our daily lives that reduce our individual carbon footprints.

On the other hand, economists understand that if you want to see a large scale change in behaviour, voluntary actions by individuals alone are not going to accomplish the overall shift in behaviour that we wish to see. We need to incentivise that behaviour, we need policies that will help move us away from our burning of fossil fuels towards renewable energy and towards more carbon friendly practices in general. And to get those policies well we need politicians who be willing to do what's right for us, rather than what might be expedient for the special interests who fund their campaigns.

We can put pressure on our policymakers to support policies to incentivise a shift away from fossil fuels and there are many forms that can take, a carbon tax, explicit subsidies for renewable energy, and there's a worthy political debate to be had about what policies we should put in place to accomplish that shift away from fossil fuels toward renewable energy.

But we need our policymakers to represent our interests and not the special interests. And the way we do that is by putting pressure, collectively, on our policymakers by voting in elections, by speaking out, by engaging in civic actions. Anything that we can do to try to make sure that climate change becomes a focus of our larger political discourse.

Chapter 8.4 concludes the course by explaining why there is hope that we can tackle climate change and what we need to do moving forward.

Okay so we've reached the end of the course. I hope you now have a clearer understanding of the science behind climate change. We've talked about the principles of atmospheric science and the complexities of the climate system. We've discussed how climate data is collected and the trends that it yields. We've learned how to do basic computations and to use theoretical models of the climate system to address questions about future climate change. We've explored the impacts of various greenhouse gas emission scenarios. Most importantly we've looked at how climate change impacts broader social, economic, and environmental issues. So it's critical to understand the science of climate change to understand the risks that we face and the actions that are necessary. So what's the goal we're working toward? Well as we've seen the science, particularly the science of climate change impacts tells us that once we warm the planet beyond about two degrees Celsius relative to pre-industrial time, we're likely to see the most damaging and potentially irreversible climate changes.

Two degrees Celsius is probably a line that we don't want to cross. That means that policies ideally should be aimed at keeping warming below that dangerous two degrees Celsius level of warming. Now the Paris agreement, as we discussed earlier in the course, gets us about halfway there. It gets us halfway from business as usual warming of the planet which is taking us towards four, five degrees Celsius warming by the end of the century, it gets us halfway to stabilise warming below that two degree mark which means that we have to make good on the Paris commitments, the countries of the world have to keep their commitments as agreed upon at the Paris summit. But we've got to go beyond Paris. We will need to ratchet up those commitments in the years ahead if we are to pursue a path that will keep warming below that dangerous two degree level. So what is in fact necessary to stabilise planetary warming below two degrees Celsius at this point?

Well in 2014 through 2016, we saw carbon emissions plateau which is the first step in bending that curve downward which is essential if we're going to keep CO2 levels below 450 parts per million and give ourselves a good chance of limiting warming below 2 degrees Celsius. Unfortunately, in 2017 and 2018 we actually saw carbon emissions tick up a little bit. What that means now, is that we've got to bring them down even more dramatically in the years ahead. We now have to bend that curve downward, we have to see a decrease in carbon emissions in the year ahead of anywhere from 3 to 4 to 5 percent per year. And obviously we are going to need policy incentives that help guide us on that pathway.

Now I often encounter among people who are concerned about climate change an increasing amount of despair and pessimism. There's a little bit too much of that, and there is this notion sometimes that this problem is too big that there's no way we can tackle it. But the science is actually very empowering in that sense because the science tells us that there is still an opportunity. The limitations at this point in keeping warming below catastrophic levels aren't in the physics of the climate, the challenges are in the politics and the policies. And so at this point we need to focus on doing what's necessary from a policy standpoint to achieve the reductions that are still possible in keeping warming below dangerous levels.

Yes there's great urgency to climate action today, but there's also reasons for hope that we can tackle this problem.

About the course lecturer: Michael Mann, Distinguished Professor of Atmospheric Science at Penn State University, a world-renowned climate expert. He received his undergraduate degrees in Physics and Applied Math from the University of California at Berkeley, an M.S. degree in Physics from Yale University, and a Ph.D. in Geology & Geophysics from Yale University. Dr. Mann was a Lead Author on the Observed Climate Variability and Change chapter of the Intergovernmental Panel on Climate Change (IPCC) Third Scientific Assessment Report in 2001 and was organising committee chair for the National Academy of Sciences Frontiers of Science in 2003. He has received a number of honours and awards including NOAA's outstanding publication award in 2002 and selection by Scientific American as one of the fifty leading visionaries in science and technology in 2002. He contributed, with other IPCC authors, to the award of the 2007 Nobel Peace Prize. He was awarded the Hans Oeschger Medal of the European Geosciences Union in 2012 and was awarded the National Conservation Achievement Award for science by the National Wildlife Federation in 2013. He made Bloomberg News' list of fifty most influential people in 2013. In 2014, he was named Highly Cited Researcher by the Institute for Scientific Information (ISI) and received the Friend of the Planet Award from the National Center for Science Education. He received the Stephen H. Schneider Award for Outstanding Climate Science Communication from Climate One in 2017, the Award for Public Engagement with Science from the American Association for the Advancement of Science in 2018 and the Climate Communication Prize from the American Geophysical Union in 2018. In 2019 he received the Tyler Prize for Environmental Achievement. He is a Fellow of the American Geophysical Union, the American Meteorological Society, the Geological Society of America, the American Association for the Advancement of Science, and the Committee for Skeptical Inquiry. He is also a co-founder of the award-winning science website RealClimate.org 

Link to Warming Stripes