Our planet’s oceans, forests, and soils perform a valuable service, absorbing half of our carbon dioxide emissions. But the more that our planet warms, the more these so-called “carbon sinks” weaken in their ability to perform this service. If we continue our current trajectory of high emissions of greenhouse gases, by the next century not only will oceans and forests absorb less carbon dioxide, but they could also even reverse their role and become carbon sources.
This is one of the key messages from Working Group I of the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), released at the beginning of August this year.
Four climate scientists from Lawrence Berkeley National Laboratory (Berkeley Lab) were key contributors to this report, which focused on the physical science basis of climate change. Reports from Working Groups II and III, which focus on adaptation and mitigation, respectively, will be out next spring.
“Under future scenarios in which we emit more carbon, those carbon sinks become less effective, in the sense that a smaller fraction of the emitted carbon is absorbed and therefore more stays in the atmosphere,” said Berkeley Lab scientist Charles Koven, a lead author for the report’s chapter on global carbon cycles. “It underscores the urgency to start acting soon. The earlier we’re able to get to net-zero carbon emissions the better. We can still keep climate change below the goals set by the Paris Agreement, but only if we act very, very fast.”
The Paris Agreement, an international treaty on climate change signed by 196 parties in 2015, seeks to limit global warming to well below 2 degrees Celsius, preferably to 1.5 degrees Celsius, above pre-industrial levels. The next UN Climate Change Conference will take place in Glasgow in November, organized by the United Nations Framework Convention on Climate Change (UNFCCC).
Other Berkeley Lab contributors to the Working Group I report were Michael Wehner, a lead author of the chapter on extreme weather, and William Collins and Chaincy Kuo, a lead author and chapter scientist, respectively, of the chapter on short-lived climate forcers, so-called because they stay in the atmosphere for only a few days or weeks to a few years, compared to carbon dioxide, the main greenhouse gas, which can persist for 100 years or more.
“The report will serve as valuable input to the UNFCCC meeting in Glasgow, as well as to governments around the world who are actively formulating plans for mitigating their contributions to anthropogenic climate change,” said Collins, head of Berkeley Lab’s Climate and Ecosystem Sciences Division.
Short-lived, but still harmful
This Sixth Assessment is the first time in which short-lived climate forcers – which include substances such as aerosols, methane, black carbon, ozone, and carbon monoxide, many of which are also air pollutants – have had their own chapter in an IPCC report. Although they may by short-lived, some of them, such as methane, are far more potent at trapping heat than carbon dioxide.
The flip side is, it may be easier to reduce their emissions, Collins noted. “There’s increasing emphasis and interest both in improving air quality and controlling climate change by reducing the emissions of these short-lived climate forcers,” he said. “This is sort of a stop-gap measure. If you reduce emissions of methane, for example, it buys you time to deal with the elephant in the room, which is carbon dioxide.”
These short-lived climate forcers predominantly contribute to warming the atmosphere, although some actually have cooling effects, which masks the strength of the warming caused by greenhouse gases. The emissions of short-lived climate forcers, especially methane, are important drivers of the current rate of warming, and while CO2 also affects near-time warming, it dominates in the long term. Methane concentrations have increased by about 3.5% since the last IPCC Assessment Report in 2013.
Another potent short-lived climate forcer is hydrofluorocarbons, or HFCs, which are commonly used as refrigerants, such as in air-conditioning. Almost 40% of the warming caused by residential and commercial emissions are from HFC emissions. “They have a very high global-warming potential,” said Kuo, also a contributing author on the chapter. “That’s a concern with global warming, with more people buying air conditioners in regions where they typically weren’t needed, such as the Pacific Northwest in the United States.”
The problem is more than academic for Kuo. “With increased fires and smoke, we have to keep the windows closed, and last year we were getting really hot inside,” she said. “I’m in Oakland and have never had to have AC before. I’m trying to figure out a solution for our home that would avoid these chemicals, which basically leak into the atmosphere, either from your home or car air conditioner.”
She added, “A key message from this chapter is that immediate, strong, and persisting emissions reductions will improve air quality and greenhouse gas effects in only a few years. In doing so, the surface temperature trends, which are currently rising, will change to decreasing or stabilize after about 20 years. It’s important to note that while carbon dioxide removal technologies can alleviate CO2 emissions, it can’t replace the need for emissions reductions.”
Also short-lived: Pandemic’s impact on climate
The scientists for this chapter were also asked to examine the climate and air quality impacts of pandemic shutdowns. “There was discernible improvement in air quality in many regions, but changes in both regional and global climate are virtually undetectable,” Collins said. “Global CO2 emissions dropped in 2020 by 7% compared to 2019, we think because of the transportation sector, but these are not long-lasting changes. COVID didn’t buy us much, is the bottom line.”
Hurricanes & heat waves
The chapter on extreme weather is of particular interest in this summer of record-shattering heat, floods, and other severe weather. A key message is that human influence is making many types of extreme climate events, including heat waves, heavy rainfall, and droughts, more frequent and severe.
“The heaviest storms have become more intense and the most severe heat waves have become hotter,” said Wehner. “And as greenhouse gas emissions continue unabated, we expect yet more of the same.”
Indeed, the report assesses with high confidence that the extreme rain rate increases with warming. In some cases, it will exceed what is expected from simple thermodynamic scaling arguments.
The report finds that scientists have made significant progress in understanding how climate change affects hurricanes. The authors find with high confidence that wind speeds of the strongest hurricanes will increase, and that the fraction of hurricanes (also called tropical storms) that reach categories 4 or 5 will increase.
One new addition to this report is event attribution, an emerging area in which scientists analyse the extent to which human influence is responsible for a particular weather event, and which is one of Wehner’s areas of expertise. In 2017 he published a paper on Hurricane Harvey, which found that the rainfall was as much as 38% heavier than it would have been in a world without global warming.
“We find event attribution to be useful and relevant information in understanding how climate change affects extreme weather, and it then furthers our confidence in our projection of future climate change,” he said.
For his chapter on global carbon cycles, Koven helped to examine a range of scenarios. In the best-case future scenarios considered by the IPCC, if the world can achieve negative emissions, Earth’s ecosystems could start to return to what they looked like in pre-industrial times, where oceans are less acidic. “There are many pathways we can choose,” he said. “If we delay things, the scale of negative emissions that would be required to stabilize warming at lower levels are mind-bogglingly large.”