IPCC : The State of the Knowledge on Climate Change

Image credit : Alisa Singer. Changing.

The Intergovernemental Panel on Climat Change (IPCC) has released its report on the state of the knowledge on climate change this Monday, 9th of August. The IPCC does not produce new scientific results, but rather compiles a vast number of studies from scientists all across the world to make a report that summarizes these results. The 2021 report gives a frightening warning: now is the last chance for humanity to reverse (some) harmful consequences of climate change. This blog post summarizes the Summary for Policymakers. There is three parts to this post: the current state of the climate, the predictions for the future and the ways to limit climate change.

The Current State of the Climate

"It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occured."

The report makes numerous claims on the current state of the climate. Among them, it is said that the increase in emission of greenhouse gases is caused by human activities, with an extremely high confidence level. The increase in greenhouse gas emissions has warmed the climate, and it is said that the last four decades have been successively warmer than the last one, and each of those decades has been warmer than any decade since 1850.

The planet gets warmer; the report claims that the planet warmed faster in the last 50 years than in any other 50-year period since the last 2000 years. It is estimated that ocean warming represents 91% of the heating in the climate system, while land warming, ice loss, and atmospheric warming accounted for 5%, 3% and 1% respectively. The report also states that the warming is slowed due to human emissions of aerosols, which reflects solar light back to outer space.

The warming of the oceans has caused sea levels to rise. Indeed, sea levels have risen by 0.2 meters since 1901; such a rise is unique in the last 3000 years. Thermal expansion explains 50% of the rise, while ice loss from glaciers represents 22%, melt of ice sheets 20% and changes in water storage 8%.

Human emissions of CO2 are the primary reason identified by the IPCC to explain the acidification of oceans. There is also a high level of confidence that oxygen levels have dropped in numerous ocean regions since the 1950s. The IPCC states that CO2 emissions in the last decade have been higher than at any given time since 1750, with annual averages of 410 ppm.

Furthermore, with the world getting warmer, glaciers melt at a very fast pace. The report states that human influence is very likely the main cause for the retreat of glaciers since the 1990s and the decrease in Arctic Sea ice. Moreover, the Arctic Sea ice has reached its lowest level in the last decade since 1850. Observations show that the retreat of the glaciers is almost synchronous in all regions of the world. Such phenomenon has not happened in the past two millenniums.

These perturbations across the globe lead to ecological disasters. It is estimated that the proportion of major cyclones has increased in the past four decades. The latitude at which the cyclones reach their peak has shifted northward. While it is not clear if human activities are responsible for this phenomenon, there is a high level of confidence that human-induced climate change is responsible for the increase in heavy precipitation during these events. The report also claims that evidences suggesting changes in extremes in every regions of the world (heatwaves, heavy precipitation, droughts, tropical cyclones) have strengthened since the last IPCC report.

Finally, the report estimates that global warming equilibrium should sit between 2°C (high confidence) and 5°C (medium confidence). The best estimate is a global increase of 3°C.

Predictions for the future

5 scenarios have been modelized to explore the possibilities of the climate response to greenhouse gas emissions. The scenarios vary from very low emissions to very high emissions of GHG. As seen in the figure below, only the scenario with very low emissions of GHG predict a warming below 1.5°C, like it was proposed by the 2015 Paris Agreement. The moderate scenario predicts a warming between 2.1°C and 3.5°C, while the very high emissions scenario predicts a warming that ranges from 3.3°C to 5.7°C.

Every additional increment of global warming will cause extremes to become more frequent and severe. For instance, it is said that for every additional 0.5°C of global warming, events such as hot extremes, heatwaves, heavy precipitation and droughts will become more frequent.

Sea levels are expected to be committed to rise for the next thousand years, even with a very low emissions scenario. The sea levels are expected to rise by 2 to 3 meters in the next 2000 years if global warming is kept under 1.5°C, but could reach 22 meters if global warming reaches 5°C.

The water cycle is expected to intensify at a global scale. Episodes of heavy precipitation and flooding should occur more often in every region of the world. Cyclones of categories 4 and 5 should increase in number annually and their wind speed should peak at new highs.

Regarding the cryosphere, global warming is expected to further amplify permafrost thawing, further increase the loss of seasonal snow cover and Arctic Sea ice. The Arctic is expected to be ice-free at least once in September before 2050. Mountain glaciers are committed to continue melting for centuries. These changes are irreversible on a centennial time scale.

Limiting future warming

The report concludes by reaffirming that there is a near-linear relationship between human CO2 emissions and global warming. The best estimate provided assess that for every 1000 GtCO2, surface temperature increases by 0.45°C. The report states that to limit human-made global warming, it is strictly necessary to reach zero net emissions of CO2, along with reducing other emissions of greenhouse gases, such as CH4.

There is a hope to remove durably store human-released CO2 from the atmosphere into reservoirs. Anthropogenic CO2 removal leading to net negative emissions could have the potential to reverse oceans acidification. However, some processes can not be reversed, such as rising sea levels.

The report also gives a warning that some events could mask the true reach of global warming. Based on statistics and historical data, scientists likely expect one large explosive volcanic eruption to occur in the 21st century. An eruption of this magnitude would cause the global temperature to drop slightly for one to three years and alter the water cycle. It is also said that under very low and low emissions scenarios, climate response to global warming could be masked by natural variability.

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Source : IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.