Global warming has “unequivocally” been caused by human activities and many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level, climate scientists have warned. According to the latest report launched by the Intergovernmental Panel on Climate Change (IPCC) on August 9th, the scale of recent changes are unprecedented over many centuries to many thousands of years. However, there is still a chance of at least reducing the impact of climate change. “Stabilizing the climate will require strong, rapid, and sustained reductions in greenhouse gas emissions, and reaching net zero CO2 emissions. Limiting other greenhouse gases and air pollutants, especially methane, could have benefits both for health and the climate,” said IPCC Working Group I Co-Chair Panmao Zhai. “This report is a reality check,” added Co-Chair Valérie Masson-Delmotte. “We now have a much clearer picture of the past, present and future climate, which is essential for understanding where we are headed, what can be done, and how we can prepare.”

The Working Group I report is the first instalment of the IPCC’s Sixth Assessment Report (AR6) and was prepared by 234 authors from 66 countries. The report of this working group deals with the physical science basis of climate change and its “Summary for Policymakers” was approved on August 6th by 195 member governments of the IPCC in a virtual session that was held over the previous two weeks. The reports of Working Group II, dealing with impacts, adaptation and vulnerability; and Working Group III, dealing with the mitigation of climate change, will be finished and published in 2022. “This report reflects extraordinary efforts under exceptional circumstances,” said IPCC Chair Hoesung Lee. “The innovations in this report, and advances in climate science that it reflects, provide an invaluable input into climate negotiations and decision-making.” Governments from 197 countries will meet this November in Glasgow for the UN climate conference Cop26.

The new report dispels any doubts as to the scale of recent changes and the role humans play in accelerating global warming. “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 occurred,” the report warns. Unless there are immediate, rapid and large-scale reductions in greenhouse gas emissions, limiting warming to close to 1.5°C or even 2°C will be beyond reach, the scientists are clear. Emissions of greenhouse gases from human activities are responsible for around 1.1°C of warming since 1850-1900, the report finds, and if this is averaged over the next 20 years, global temperatures are expected to reach or exceed 1.5°C of warming. “Climate change is already affecting every region on Earth, in multiple ways. The changes we experience will increase with additional warming,” said Zhai. “For example, every additional 0.5°C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes, including heatwaves (very likely), and heavy precipitation (high confidence), as well as agricultural and ecological droughts in some regions (high confidence),” according to the Summary for Decisions Makers. At 2°C of global warming, heat extremes would more often reach critical tolerance thresholds for agriculture and health.

The climate experts also highlight that climate change is not just about temperature but also brings multiple different changes that will differ from region to region. These include changes to wetness and dryness, to winds, snow and ice, coastal areas and oceans. For example, climate change will also intensify the water cycle. This can lead to more intense rainfall and associated flooding, as well as more intense drought in many regions. Rainfall patterns will be affected: In high latitudes, for example, precipitation is likely to increase, while it is projected to decrease in large parts of the subtropics. In coastal areas, continued sea level rise throughout the 21st century will be a problem that contributes to more frequent and severe coastal flooding in low-lying areas and coastal erosion. In addition, global warming will lead to increased permafrost thawing, and the loss of seasonal snow cover, melting of glaciers and ice sheets, and loss of summer Arctic sea ice. Changes in the quantity and seasonality of water due to climate change will heavily influence the food security and economic prosperity of many countries, particularly in the arid and semi-arid areas of the world including Asia, Africa, Australia, Latin America, the Mediterranean, and small island developing states, the report warns. According to the full report, whose final text remains subject to revisions following the approval on Friday, “having too much or too little water increases the likelihood of flooding and drought, as precipitation variability increases in a warming climate.” The authors warn that “changes in precipitation and glacier runoff and snowmelt influence other hydroclimate variables like surface and subsurface runoff, and groundwater recharge, which are critical to the water, food and energy security of many regions.” (ab)