The Intergovernmental Panel on Climate Change (IPCC) yesterday released the IPCC Working Group 1 report, “Climate Change 2021: The Physical Science Basis” (the Report), part of the Sixth Assessment Report, providing an updated assessment of the physical understanding of the current state of the climate system and climate change. The Report predicts that global temperatures are likely to continue to increase beyond the 1.5-2°C target of the Paris Agreement without widespread and steep global reductions in greenhouse gas (GHG) emissions. This bulletin summarizes the Report’s key findings.
The current state of the climate. The Report reiterates that the warming of the atmosphere, oceans, and land are human-caused, with rapid changes being observed in the atmosphere, ocean, cryosphere, and biosphere. In addition, the Report confirms that anthropogenic climate change is globally affecting weather and climate extremes, with increased heatwaves, heavy precipitation, droughts, and tropical cyclones more readily attributed to human influence.
Possible climate futures. According to the Report, under all emissions scenarios, global surface temperatures will continue to increase until mid-century, with temperatures predicted to exceed 1.5-2°C this century without deep reductions of GHGs. As the climate warms, changes in climate systems will become larger, increasing the frequency and intensity of hot extremes, marine heatwaves, heavy precipitation, droughts, intensity of tropical cyclones, and reductions in Arctic sea ice, snow cover, and permafrost. The Report indicates that changes in the ocean, ice sheets, and global sea levels, resulting from past and future GHG emissions, will likely be irreversible for hundreds of years.
Climate information for risk assessment and regional adaptation. The Report indicates that all regions are expected to increasingly experience concurrent and multiple changes in climatic impact-drivers amplified at 2°C compared to 1.5°C, with greater increases at even higher global temperatures. The Report also indicates that even “low-likelihood” outcomes such as ice sheet collapse and abrupt ocean circulation changes must still form part of risk assessment.
Limiting future climate change. The Report indicates that limiting anthropogenic global warming requires limiting cumulative CO2 emissions, reaching net-zero CO2 emissions, and reducing other GHG emissions. Reductions in CH4 emissions is likely to also limit warming effects and improve air quality. Importantly, the Report notes that scenarios with low GHG emissions would rapidly and sustainably limit many climate impact-drivers within 20 years, including the frequency of sea level events, heavy precipitation and flooding, and heatwaves.