The latest landmark climate science report goes much further than previous ones in providing estimates of how bad things might get as the planet heats up, even if a lack of data may mean it underestimates the perils.
Scientists have used the seven years since the previous assessment report of the Intergovernmental Panel of Climate Change (IPCC) to narrow the uncertainties around major issues, such as how much the planet will warm if we double atmospheric levels of carbon dioxide and other greenhouse gases.
While temperatures have risen largely in lockstep with rising CO2, this IPCC report examines in much more detail the risks of so-called abrupt changes, when relatively stable systems abruptly and probably irreversibly shift to a new state.
Michael Mann, director of the Pennsylvania State University’s Earth System Science and one of the world’s most prominent climate researchers, says the models are not capturing all the risks as the climate heats up.
Perhaps the most prominent of these threats is a possible stalling of the Atlantic Meridional Overturning Circulation (AMOC). Also known as the Gulf Stream, it brings tropic water north from the Caribbean, keeping northern Europe much warmer than its latitude might otherwise suggest, and threatening massive disruptions if it slows or stops.
“Where the models have underestimated the impact is with projections of ice melt, the AMOC, and – I argue in my own work – the uptick on extreme weather events,” Professor Mann tells the Herald and The Age.
Stefan Rahmstorf, head of research at the Potsdam Institute for Climate Impact Research, agrees that climate models have not done a good job of reproducing the so-called cold blob in the subpolar Atlantic that is forming where melting Greenland ice is cooling the subpolar Atlantic.
If they are not picking that blob up, “should we trust those models on AMOC stability?” Professor Rahmstorf asks.
The IPCC’s language, too, doesn’t necessarily convey the nature of the threat, much of which will be detailed in the second AR6 report on the impacts of climate change, scheduled for release next February.
“Like just stating the AMOC collapse by 2100 is ‘very unlikely’ – that was in a previous report – it sounds reassuring,” Professor Rahmstorf said. “Now the IPCC says they have ‘medium confidence’ that it won’t happen by 2100, whatever that means.”
West Antarctic melt
Another potential tipping point is the possible disintegration of the West Antarctic ice sheet. Much of the sheet lies below sea level and as the Southern Ocean warms, it will melt causing it to “flow” towards the sea in a process that is expected to be self-sustaining.
This so-called marine ice sheet instability is identified in the IPCC report as likely resulting in ice mass loss under all emissions scenarios. There is also “deep uncertainty in projections for above 3 degrees of warming”, the report states.
Containing enough water to lift sea levels by 3.3 metres, it matters what happens to the ice sheet. As Andrew Mackintosh, an ice expert at Monash University, says, the understanding is limited: “We know more about the surface of Mars than the ice sheet bed under the ice.”
Permafrost not so permanent
Much has been made about the so-called “methane bomb” sitting under the permafrost in the northern hemisphere. As the Arctic has warmed at more than twice the pace of the globe overall, with heatwaves of increasing intensity and duration, it is not surprising that the IPCC has listed the release of so-called biogenic emissions from permafrost thaw as among potential tipping points.
These emissions could total up to 240 gigatonnes of CO2-equivalent which, if released, would add an unwanted warming boost.
The IPCC lists as “high” the probability of such releases during this century, adding there is “high confidence” that the process is irreversible at century scales.
“In some cases abrupt changes can occur in Earth System Models but don’t on the timescales of the projections (for example, an AMOC collapse),” said Peter Cox, a Professor of Climate System Dynamics at the UK’s University of Exeter. “In other cases the processes involved are not yet routinely included in ESMs [such as] CO2 and methane release from deep permafrost.”
“In the latter cases IPCC statements are made on the basis of the few studies available, and are necessarily less definitive,” he said.
From the Amazon rainforest to the boreal forests of Russia and Canada, there is a risk of fire and pests that could trigger dieback and transform those regions.
Australia’s bush faces an increased risk of bad fire weather days right across the continent, the IPCC notes. How droughts, heatwaves and heavy rain and other extreme events will play out at a local level is also not well understood.
Ocean acidification and marine heatwaves also mean the world’s coral reefs will be much diminished at more than 1.5 degrees of warming. “You can kiss it goodbye as we know it,” says Sarah Perkins-Kirkpatrick, a climate researcher at the University of NSW, said.
Global monsoons, which affect billions of people including those on the Indian subcontinent, are likely to increase their rainfall in most parts of the world, the IPCC said.
Andy Pitman, director of the ARC Centre of Excellence for Climate Extremes, said policymakers need to understand that much is riding on these tipping points not being triggered as even one or two of them would have long-lasting and significant effects. “How lucky do you feel?” Professor Pitman says.
The Biggest uncertainty
Christian Jakob, a Monash University climate researcher, said that while there remain important uncertainties, science is honing most of those risks down.
Much harder to gauge, though, is which emissions path humans are going to take. Picking between the five scenarios ranging from low to high that we are going to choose is “much larger than the uncertainty we have in the science,” Professor Jakob said.