Climate Summary

It took only 120 years for humans to alter the planet's climate, if the latest report of the Intergovernmental Panel on Climate Change (IPCC) Working Group I on the "Physical Science Basis of the Sixth Assessment" (released August 9th, 2021) is to be trusted.?

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The planet of today is hardly recognizable or comparable to the one that existed during 1850-1900. The planet's climate was not influenced by humans in the pre-industrial period. IPCC measures the level of human-emitted greenhouse gases (GHG) that cause global warming and the resultant change in the climate through this period. The latest IPCC report reads like a self-inflicted death sentence: Extreme weather events to change in seasons to rising sea levels have convicted humans. "The IPCC Working Group 1 Report is a code red for humanity", said United Nations Secretary-General António Guterres. It is a feedback loop mechanism where a warming atmosphere leads to fundamental changes. In the ways, the planetary climate operates. Climate zones have shifted poleward in both hemispheres, and the growing season has on average lengthened by up to two days per decade since the 1950s in the Northern Hemisphere extra-tropics. The warmer the atmosphere, the greater it is capacity to carry moisture. In simple terms, global warming leads to increased rainy days, and of higher intensity. With a 1°C increase in temperature, there is a 7 percent increase in the intensification of rains. Storm systems have undergone several changes in their tracks. Mid-latitude storm tracks have likely shifted poleward in both hemispheres since the 1980s, with marked seasonality in trends. For the Southern Hemisphere, human influence very likely contributed to the poleward shift of the closely related extratropical jet in austral summer.

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The rise in the average surface temperature of the Earth will cross 1.5 degrees Celsius in the next 20 years, and 2°C by the middle of the century (without a sharp reduction of GHG emissions). There will be further warming in the coming decades unless there are immediate, strong, and rapid reductions in global emissions. So even if emissions are brought to net-zero by the mid-21st-century, there will be an "overshoot" of the 1.5°C limits by 0.1°C. The threshold of 2°C will be exceeded during the 21st century without deep emission cuts, according to IPCC. In the first two decades of the 21st century, the average global surface temperature was 0.99°C higher than the 1850-1900 level. It was 1.09°C higher than the historic base period in 2011-2020, indicating faster warming of the planet. The temperature reported in this decade was last observed some 6,500 years ago, in the Middle Holocene period that was warmer than the present day. The estimated increase in global surface temperature since AR5 (Assessment Report 5, last released in 2013) is principally due to further warming since 2003-2012 (adding +0.19°C) [as per scientists in the IPCC report that synthesized over 14,000 scientific reports]. Other recent reports have indicated that the world may cross the 1.5°C mark in the next five years for individual months or years. Though this would not translate to the world crossing the 1.5°C mark, it does indicate the unprecedented levels of warming that humans are causing.?

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The last time similar warming occurred was 1,25,000 years ago. This was analyzed from paleoclimate records going back millions of years. If the GHG emissions are not drastically reduced and global temperature rise above pre-industrial levels crosses the 2.5°C mark, the planet would be as warm as it was three million years ago when the climate and associated animal and plant species were very different.

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Global warming is melting the planet's frozen landmass, the Polar Regions. In 2011-2020, the annual average Arctic sea ice area reached its lowest level since at least 1850. The global nature of glacier retreat, with almost all of the world's glaciers retreating synchronously, since the 1950s is unprecedented in at least the last 2,000 years. A scary forecast is that the Arctic is likely to be sea-ice-free in September at least once before 2050.

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The rise in global temperature follows the same trajectory in the emission of greenhouse gases. Since 2011, concentrations (of greenhouse gases) have continued to increase in the atmosphere, reaching annual averages of 410 parts per million for carbon dioxide (CO2), 1886 parts per billion for methane (CH4), and 332 parts per billion for nitrous oxide (N2O) in 2019. In April 2021, the CO2 concentration in the atmosphere reached a monthly level of 416 parts per million, the highest that they have been in two million years. Observed increases in well-mixed GHG concentrations since around 1750 are unequivocally caused by human activities.?

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The report attributed this increase in human-emitted GHGs to the rise in global temperature. It has, in fact, put a definitive figure to temperature rise attributed to human-induced GHG emissions. Human responsibility - The likely range of total human-caused global surface temperature increase from 1850-1900 to 2010-2019 is 0.8°C to 1.3°C, with the best estimate of 1.07°C. The breakup - It is likely that well-mixed GHGs contributed warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural drivers changed global surface temperature by -0.1°C to 0.1°C, and internal variability changed it by -0.2°C to 0.2°C.?

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The only chance of limiting the temperature rise to below 1.5°C is if the low-emission scenario is undertaken, for which countries would have to raise their emission reduction targets under the Paris Agreement of 2015. The harsh reality of climate change is playing out in real-time before our eyes. It is a foretaste of what faces future generations.?

- Aindri Abhishek Singh

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