August 9, Intergovernmental Panel on Climate Change (IPCC) publishes its most comprehensive report on the science of climate change since 2013. This is the first of four reports published under the last IPCC assessment cycle, with subsequent reports due in 2022.
Over the past eight years, climatologists have improved the methods they use to measure different aspects of the climate and to model (or project) what might happen in the future. They also observed the changes that took place before our eyes.
The updated assessment comes three months before world leaders meet in Glasgow, Scotland, to find ways to avoid the worst effects of climate change and renew their commitments to cut greenhouse gases. It also comes in the middle of a new year marked by intense heat waves, droughts, forest fires, floods and storms.
The report will provide policymakers with the best available information regarding the science of climate change. This is essential for long-term planning in many sectors, from infrastructure to energy to the well-being of society.
Here are five things to watch out for in the new report:
1. How sensitive is the climate to increasing carbon dioxide?
Levels of carbon dioxide (CO2) in the atmosphere are higher today than they were in 800 years, reaching 419 parts per million (ppm) in May 2021. The average temperature of the planet increases with each increase in the concentration of CO2 in the atmosphere, but the magnitude of this increase depends on many factors.
Climatologists use models to understand the magnitude of the warming that occurs when CO concentrations2 double compared to pre-industrial levels - from 260 ppm to 520 ppm - a concept called "Climate sensitivity". The more sensitive the climate, the faster greenhouse gas emissions must be reduced to stay below 2 C.
(Data: IPCC, Graph: Alex Crawford)
Older climate models estimated that a doubling of CO2 atmospheric temperature would lead to an increase in the 2,1 C to 4,7 C. The latest climate models, called CMIP6 (for "intercomparison project of coupled models"), widened the range between 1,8 C to 5,6 C, which means that the climate is at least as sensitive to a doubling of carbon dioxide as previous models showed, but that it could, in fact, be even more.
This range is influenced by uncertainties associated with a number of factors, including water vapor and cloud cover, and how they will increase or decrease the effects of warming. Scientists are working to narrow the range of climate projections to better understand how quickly we need to reduce greenhouse gas emissions to avoid the worst effects of climate change and adapt to others.
2. What is happening with the clouds?
The clouds reflect about one quarter of incoming sunlight far from Earth. So, if greater warming results in increased clouds, more sunlight can be expected to be reflected, slowing the warming. However, clouds also insulate the Earth, by retaining the heat given off by the surface. Thus, the increase in cloud cover (for example during the night) could amplify the warming.
Two questions arise First, many factors, including cloud type, altitude and season, determine the overall effect of a cloud on warming. Then clouds are extremely difficult to model: how the models will account for these factors is the key to assessing the range of climate sensitivity.
3. Has climate change fueled recent extreme weather events?
Since the last IPCC report, our ability to assess the impact of global warming on extreme events has improved significantly. Chapter 11 of the last report is devoted to it.
Warmer air can hold more water. This can lead to greater evaporation from the land and lead to droughts and forest fires. In addition, an atmosphere containing more water can produce more precipitation and flooding.
4. Have regional climate projections improved?
(Alex Crawford), CC BY-SA
Climate models assessed by the IPCC are global models. This is essential for understanding the links between the tropics and the poles or between land and ocean. However, this comes at a cost: Models find it difficult to simulate many items smaller than 100 kilometers in size, such as small islands, or events like small storms.
Since the last IPCC report, techniques for exploiting this information on a large scale and refining it showed how the regional and local climate has changed and could change in the future. Other experiences focus on regional issues, such as repercussions of the loss of arctic sea ice on storms.
5. How will the Antarctic ice caps contribute to sea level rise?
But the biggest potential source of sea level rise over the next century is Antarctica. The models for the ice caps show that their melting in Antarctica will add between 14 and 114 centimeters to sea level rise by 2100. This is a huge range, and it all depends on whether the West Antarctic ice sheet remains relatively stable or if it starts to collapse slowly but surely.
The way the IPCC communicates its scientific knowledge will have an impact on the way in which coastal communities plan for sea level rise. Low-lying towns, such as Lagos, Nigeria, could become uninhabitable by the end of the century due to rising sea levels, especially if higher model estimates are correct.
The IPCC report will give policymakers a better understanding of how climate change affects us today. This will be particularly useful for putting in place short-term coping strategies.
But as science improves, the outlook for climate change grows bleaker. The great uncertainties that remain mean that climate scientists have a lot of work ahead of them.
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