Last week, the Royal Society, advancing knowledge since 1662, hosted a two day conference on “Next Steps in Climate Science” following the release of the IPCC draft Physical Science report. The conference gathered together climate scientists to explore the level of understanding and confidence in key parts of the science, and to discuss where research should go next. As well as Professors of Climate Science from the UK’s leading universities, the German, French and UK Meteorological Offices were represented, and various international climate scientists.
NGOs were also invited to deepen their understanding of the science, and Sandbag was there. One discussion was on the chances of carbon emissions causing abrupt and irreversible changes this century; something we’d all like to know. This lecture did not focus on temperature rises as a whole, which are projected to move the climate to an “unprecedented” climate as early as 2047if emissions continue as they are, but on the scope for changes this century which are abrupt (a few decades or less) and irreversible (on human timescales).
Professor Mat Collins, the Joint Met Office Chair in Climate Change at Exeter University, led the lecture. There were definite positives; in general the modelling is showing linear responses, and the IPCC is not confident that any of the abrupt changes studied will happen before 2100, other than summer Arctic sea ice loss. However, many of these processes will begin or already have begun, and are often irreversible, in that the time for them to recover is geologic. The studies make clear that it is difficult to use a relatively short data set to estimate probabilities for events which are very rare in the Earth’s history, although not being able to precisely predict the date of an abrupt change is not the same as an absence of risk. Nevertheless Professor Collins highlighted the following areas where palaeoclimate data indicates abrupt changes are possible, and summarised the state of the science for each abrupt change.
The collapse of the Atlantic Meridional Overturning Circulation This persistent ocean current which maintains the climate across Europe has been hypothesised to come to a sudden stop, as seen around the end of the last ice age. The IPCC says there is no significant trend currently, and whilst it expects weakening by 2050, it has high confidence that it will not collapse. This was one of the most controversial discussions over the two days, with some scientists commenting that the high confidence it wouldn’t collapse was unwarranted given the low confidence in the degree to which it would weaken.
Greenland or West Antarctic Ice sheets collapse Despite an accelerating volume of melt, the IPCC states that collapse of these ice sheets, sitting on land unlike the Arctic ice cap, and so capable of making large contributions to sea level rise (7 metres and 3 metres respectively), is exceptionally unlikely this century, but likely on centennial timescales.
Release of carbon from permafrost Carbon bound up in permanently frozen soils may become a small but significant net source of emissions this century, increasing the scale of emissions reductions humanity needs to make. Permafrost represents a store twice the amount of current atmospheric carbon, and loss is likely irreversible once melting occurs.
Release of methane from clathrates Very unlikely that there will be a catastrophic release this century of this strongly warming gas bound up in a compounds on the seafloor and in permafrost. However, it is likely that emissions from clathrates would increase. New science indicates sea level rise and pressure increases may help stabilise clathrates.
Tropical and boreal forest dieback Amazon desertification is in particular a threat, with some studies suggesting near complete loss of the rainforest if temperatures reach 4C, as could happen before the end of this century. However, the IPCC found projections currently have low confidence. The processes are very complex, and thus the threshold at which the forests change qualitatively is still unknown. There is possible benefit from CO2 fertilization to offset decreased equatorial rainfall.
Ice-free summer Arctic This is a story we all know; the IPCC says it’s likely it will be ice-free before 2050. Not only is sea ice extent rapidly decreasing, but sea ice volume is dropping even faster, as thick, multi-year ice is replaced by thin, single season ice. Arctic ice doesn’t contribute to sea level rise other than through thermal expansion, but the loss of the white reflecting cap will further speed the absorption of heat by the oceans.
Long-term droughts and Monsoon collapse Low confidence in models of collapse. Probably not irreversible, given past palaeoclimate droughts and their recovery. The Asian Monsoon is weakening, but this may be aerosol-caused, not temperature.
Ocean acidification/deoxygenation As the oceans absorb much of the carbon dioxide we are releasing through the burning of fossil fuels, they are becoming increasingly acidic. The IPCC did not consider this under the abrupt changes, but the report says it is likely that some oceans will become corrosive to aragonite “within a decade”, dissolving the shells of marine creatures vital for ocean health.
In summary; a variety of scenarios have been considered, each of which would cause major damage to human life. At the moment, on the main, though they know that they have happened before and could happen again, the scientists don’t know enough to confidently predict when exactly many of them would happen. The question is; for how long are we willing to continue loading the dice against ourselves, and what will it take for the drastic emissions cuts that will prevent this ongoing experiment with Earth from playing out?
For more information, see Table 12.4 in Chapter 12 of the IPCC WG1 draft Report (“Long-term Climate Change: Projections, Commitments and Irreversibility”)