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Humans produce large amounts of greenhouse gas emissions, primarily through fossil fuel burning, agriculture, deforestation and decomposing waste.
The IPCC’s projections for global average sea level rise in meters with higher-impact pathways and the level of greenhouse gas emissions.
The Gulf Stream is part of the Atlantic Meridional Overturning Circulation. A slowdown would affect temperature in Europe and sea level rise along the U.S. East coast.
IPCC Sixth Assessment Report
There is mixed evidence for abrupt changes in the polar ice sheets, but clear evidence that changes in the ice sheets can be locked in for centuries and millennia.
If the world succeeds in limiting warming to 1.5 C (2.7 F), we expect to see about 2-3 meters (7-10 feet) of sea level rise over the next 2,000 years; if the planet continues to warm and reaches a 5 C (9 F) increase, we expect to see about 20 meters (70 feet) over the next 2,000 years.
Some people also discuss summer Arctic sea ice – which has undergone substantial declines over the last 40 years and is now smaller than at any time in the past millennium – as a system with a “tipping point.” However, the science is pretty clear that there is no critical threshold in this system. Rather, summer Arctic sea ice area decreases roughly in proportion to the increase in global temperature, and if temperature were stabilized, we would expect sea ice area to stabilize also.
What do scientists know now about hurricanes that they didn’t realize when the last report was written?
Since the last IPCC assessment report in 2013, there has been increasing evidence that hurricanes have grown more intense, and intensified more rapidly, than they did 40 years ago. There’s also evidence that hurricanes in the U.S. are moving more slowly, leading to increased rainfall.
However, it’s not clear that this is due to the effects of greenhouse gases – reductions in particulate pollution have also had important effects.
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The clearest effect of global warming is that a warmer atmosphere holds more water, leading to more extreme rainfall, like that seen during Hurricane Harvey in 2017. Looking forward, we expect to see hurricane winds and hurricane rains continue to increase. It’s still unclear how the overall number of hurricanes will change.
The report involved 234 scientists, and then 195 governments had to agree on the summary for policymakers. Does that broad range of views affect the outcome?
When you’re writing a report like this, a key goal for the scientists is to accurately capture points of both scientific agreement and scientific disagreement.
For example, with respect to ice sheet changes, there are certain processes on which there is broad agreement and other processes where the science is still emerging and there are strong, discordant views. Yet knowing about these processes may be crucially important for decision-makers trying to manage risk.
That’s why, for example, we talk not only about most likely outcomes, but also about outcomes where the likelihood is low or as-yet unknown, but the potential impacts are large.
A scientist plants a flag to identify a GPS position on Greenland’s Helheim Glacier in 2019. The glacier had shrunk about 6 miles (10 kilometers) since scientists visited in 2005.
AP Photo/Felipe Dana
The IPCC uses a transparent process to produce its report – the authors have had to respond to over 50,000 review comments over the three years we’ve spent writing it. The governments also weigh in, having to approve every line of a concise Summary for Policy Makers that accurately reflects the underlying assessment – oftentimes making it clearer in the process.
I’m very pleased that, as with past reports, every participating government has signed off on a summary that accurately reports the current state of climate science.
Read more:
The water cycle is intensifying as the climate warms, IPCC report warns – that means more intense storms and flooding
Robert Kopp, Professor, Department of Earth & Planetary Sciences, and Director, Rutgers Institute of Earth, Ocean, and Atmospheric Sciences, Rutgers University
This article is republished from The Conversation under a Creative Commons license. Read the original article.