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The Sixth Assessment Report on Climate Change

Rithika Ganesan
September 2nd, 2021 · 3 min read

The Intergovernmental Panel on Climate Change (IPCC), since 1990, has released assessment reports presenting evidence of our changing climate. The first of four parts of the Sixth Assessment Report (AR6) was released last month. It is the first major review since the fifth report in 2013.

The AR6, unsurprisingly, reports that the conditions of our climate and environment are terrible, in the context of human existence, and will only get worse. While the report provides sound scientific evidence and projections of different environmental variables, the countless wildfires, earthquakes, and hurricanes that have constantly flooded the news cycles tell us all we need to know: unless immediate action is taken, we will suffer. The report lays out the details of that suffering.

Human influence on the climate has been a fully established fact since the Fifth Assessment Report (AR5) in 2013, and the evidence has strengthened steadily from the AR2 to the AR6. Every significant climate driver is associated with human activity. Not unlike lab rats repeatedly choosing to stimulate their hypothalami for instant gratification even though the process ultimately leads to death, we have chosen to continue indulging in the various conveniences that are degrading our environment.

Greenhouse gas emissions have remained the primary cause of global warming. The relation between global cumulative CO2 emissions and increase in surface temperature was confirmed to be nearly linear. Global warming levels are discussed relative to the temperatures recorded between 1850-1900, the earliest period from which reliable temperature data is available. Since the Industrial Revolution in the late 1700s, the average global temperature has risen by 1.1o C, primarily due to human activity. If there are no immediate, considerable reductions in greenhouse gas emissions made, we will surpass 2o C rapidly. The AR6 also notes a high likelihood that current levels of atmospheric CO2 have not been experienced for at least 2 million years.

A new feature of this report, the assessed future change in global surface temperature has been constructed using both scenario-based projections, which previous reports had focussed on, and observational constraints based on past simulations of warming. A central set of five scenarios, entailing both high CO2 emission pathways without any measures taken to mitigate the effects and new low CO2 emission pathways, were used. Paleoclimate evidence, which makes use of instrument-based observations of climate variables in the past, was used. However, paleoclimate archives such as glaciers, corals, and trees are also disappearing at alarming rates. Reconstructions of past climate change over different time scales were made with the available archives, showing that we are truly in a new epoch of immense rates of change, propelled by human activity.

Upper ocean salinity, global ocean temperatures, and ocean acidification have all increased considerably over the past 40 years. Marine heatwaves have also become more common and have been attributed to anthropogenic warming. Current Arctic sea ice coverage levels, throughout the year, are at the lowest since at least 1850. Even if the global temperature is stabilized, glaciers, the Greenland Ice Sheet, and the Antarctic Ice Sheet will continue to lose mass for at least the duration of this century. Consequently, sea level rises are expected to occur even if emissions cease.

In a stable climate, the amount of energy that the Earth receives from the Sun is roughly equal to how much energy is directed back to space as reflected sunlight and thermal radiation. Climate drivers, mainly greenhouse gases and aerosols, interfere with this balance. Radiative forcing is a standard of the influence that a climatic factor has on the amount of radiant energy reaching the Earth’s surface. Effective radiative forcings were calculated for aerosols, sulphur dioxide, and methane emissions, among others. The total ERF has been growing at increasing rates since the 1970s and has risen to 3.48 Wm-2 compared to an estimated 1.96 Wm-2 in 1750.

For many climate processes, we have long crossed the tipping point up to where the conditions were reversible. The report reassures us of the bleak conclusions already drawn in previous studies, and will mainly serve as climate anxiety fodder for young people while policy makers will continue to live in bliss.

Rithika Ganesan, Batch 19

Sources: https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group 15 I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Masson-Delmotte, V., 16 P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. 17 Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. 18 Zhou (eds.). Cambridge University Press. In Press.

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