The latest report from the Intergovernmental Panel on Climate Change (IPCC) – Climate Change 2021: The Physical Science Basis – reaffirms and builds upon decades of climate science, bringing together major advances in physics over the past decade. Published in August, written by a team of 234 scientists and based on evidence from 14,000 research studies, the report presents the clearest picture yet of how the Earth’s climate functions and human activities affect it.
Today, we know better than ever how the climate changed in the past, how it is changing now, and how it may change in the future. The report looks at the potential futures from the different pathways of greenhouse gas emissions.
Where are we today?
The report shows that recent changes in the climate are widespread, rapid and intensifying. And it shows that it is now an established fact that human activities are causing climate change.
On average, the Earth’s surface was 1.1°C warmer over the past decade compared to the late 19th century. Our best estimate is that all of this observed warming was caused by emissions from human activities, such as the burning of fossil fuels, deforestation and agriculture. In fact, the warming due to greenhouse gases, which is dominated by carbon dioxide (CO2) and methane (CH4), has been partly masked by cooling from pollution particles.
This 1.1°C of human-induced climate change is already affecting every region on Earth, strengthening the frequency and intensity of extreme events such as heatwaves, heavy precipitation (such as rain or snow), droughts and fire.
With further global warming, every region of the Earth is projected to experience increasing physical changes. These changes will be more widespread and pronounced with every additional fraction of global warming, and you can explore them using the report’s online interactive atlas. Some changes are unavoidable, and our improved knowledge on what to expect should inform risk management and adaptation strategies.
The physical climate changes include increases to regional trends in temperature, changes in the frequency and intensity of hot extremes, marine heatwaves, heavy precipitation, droughts, the proportion of the world’s population experiencing intense tropical cyclones, and reduced snow cover, permafrost and Arctic sea ice.
A warmer climate will intensify the water cycle and lead to increases in both very wet and very dry weather, climate events and seasons, with implications for flooding and drought.
While these aspects change in direct relationship with the level of global warming, and so can stop changing when global surface temperature stops increasing, changes involving the slow components of the climate system, such as the deep ocean, ice sheets and the resulting rise in sea levels, are irreversible for centuries to millennia.
It is certain that sea levels, which have seen accelerated rises in recent decades, will continue to rise over thousands of years, but the rate and magnitude of rise could be limited if global greenhouse gas emissions decrease sharply, thus giving more time for adaptation on coastlines worldwide.
Reduced ocean mixing, increased ocean acidification and the loss of dissolved oxygen will continue in the 21st century, but at slower rates if greenhouse gas emissions decrease. This is very important for protecting marine life and the people who depend on it.
There are still large uncertainties regarding low-likelihood outcomes, such as ice sheet collapse, abrupt ocean circulation changes, or compound extreme events (when two or more climate events occur, such as a tropical cyclone followed by a heatwave). Such events cannot be ruled out. The probability of such outcomes increases with higher global warming levels.
How to limit future climate change
The very clear finding from this report reiterates what we have known for many years: every tonne of CO2 emissions adds to global warming.
The report shows that global warming is expected to reach 1.5°C in the next 20 years. If emissions stagnate close to today’s level over a few decades, warming would exceed 1.5°C in the next 20 years, 2°C by 2050, and 3°C in the next century. However, these higher levels of warming – and their severe consequences – can be avoided if deep reductions in CO2 and other greenhouse gases occur as soon as possible and continue over the coming decades.
Limiting human-induced global warming therefore requires limiting cumulative CO2 emissions, reaching at least net-zero CO2 emissions, along with strong reductions in other greenhouse gas emissions. Strong, rapid and sustained reductions in CH4 would also limit the warming effect that is resulting from declining aerosol pollution and would improve air quality.
The good news is that we would see the impacts of reduced global greenhouse gas emissions very quickly. In just a few years, reductions in emissions would lead to discernible effects on greenhouse gas and aerosol concentrations, and air quality. The effects on global surface temperature would be seen within around 20 years. Our future climate depends on our decisions now.
Valérie Masson-Delmotte is co-chair of the IPCC Working Group I, which assesses the physical science of climate change.
This article first appeared in Spotlight’s print edition Energy and Climate Change: Cop26 and Beyond on 29 October 2021.