Time is running out
The IPCC scientists warn global warming of 2°C will be exceeded during the 21st century. Unless rapid and deep reductions in CO2 and other greenhouse gas emissions occur in the coming decades, achieving the goals of the 2015 Paris Agreement ‘will be beyond reach’.
The assessment is based on improved data on historical warming, as well as progress in scientific understanding of the response of the climate system to human-caused emissions.
‘It has been clear for decades that the Earth’s climate is changing, and the role of human influence on the climate system is undisputed. Yet the new report also reflects major advances in the science of attribution – understanding the role of climate change in intensifying specific weather and climate events.’
IPCC Working Group I co-chair
The experts reveal that human activities affect all major climate system components, with some responding over decades and others over centuries.
Scientists also point out that evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts and tropical cyclones, and their attribution to human influence, has strengthened.
They add that many changes in the climate system become larger in direct relation to increasing global warming.
This includes increases in the frequency and intensity of heat extremes, marine heatwaves and heavy precipitation; agricultural and ecological droughts in some regions; the proportion of intense tropical cyclones and reductions in Arctic sea ice, snow cover and permafrost.
The report makes clear that while natural drivers will modulate human-caused changes, especially at regional levels and in the near term, they will have little effect on long-term global warming.
A century of change
The IPCC experts project that in the coming decades climate changes will increase in all regions. For 1.5°C of global warming, there will be increasing heat waves, longer warm seasons and shorter cold seasons.
At 2°C of global warming, heat extremes are more likely to reach critical tolerance thresholds for agriculture and health.
But it won’t be just about temperature. For example, climate change is intensifying the natural production of water – the water cycle. This brings more intense rainfall and associated flooding, as well as more intense drought in many regions.
It is also affecting rainfall patterns. In high latitudes, precipitation is likely to increase, while it is projected to decrease over large parts of the subtropics. Changes to monsoon rain patterns are expected, which will vary by region, the report warns.
Coastal areas will see continued sea level rise throughout the 21st century, contributing to more frequent and severe coastal flooding in low-lying areas and coastal erosion.
Extreme sea level events that previously occurred once in 100 years could happen every year by the end of this century.
The report also indicates that further warming will amplify permafrost thawing, and the loss of seasonal snow cover, melting of glaciers and ice sheets and loss of summer Arctic sea ice.
Changes to the ocean, including warming, more frequent marine heatwaves, ocean acidification and reduced oxygen levels, affect both ocean ecosystems and the people that rely on them, and they will continue throughout at least the rest of this century.
Climate change magnified in cities
Experts warn that for cities, some aspects of climate change may be magnified, including heat, flooding from heavy precipitation events and sea level rise in coastal cities.
Furthermore, IPCC scientists caution that low-likelihood outcomes, such as ice sheet collapse or abrupt ocean circulation changes, cannot be ruled out.
The report explains that from a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative carbon dioxide emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions.
‘Strong, rapid and sustained reductions in methane emissions would also limit the warming effect resulting from declining aerosol pollution’, the IPCC scientists state.