A fascinating scientific study exploring how the planet responds to greenhouse gasforcing was recently reported in the prestigious journal Science. The research by Andreas Schmittner at Oregon University and colleagues, looked at global climate during the last ice age. The idea was to use a concept known as ‘climate sensitivity’ to try to gain some sort of insight into how the world will warm with a doubling of greenhouse gas levels.
To do this the team took a two-pronged approach. On the one hand, they used natural measures of past climate preserved in ice, lake sediments, trees and the ocean floor to reconstruct the world’s temperature 20,000 years ago. Alongside this, a series of computer model runs were also made for an ice age world with relatively low amounts of greenhouse gases; the precise levels known from bubbles trapped in ancient Antarctic ice. The climate sensitivity was tweaked in each run until the modelled temperatures best matched those found in the ice age reconstruction.
Extrapolating the results to the future appears to imply the world could be between 1.7 and 2.6˚C warmer with a doubling of carbon dioxide. As with many scientific articles on climate change, it attracted its fair share of media coverage. But the way this work was communicated does provide some fascinating insights into how scientific results are portrayed in the mainstream. Some accounts – many of which were published online – led with the reassuring message that the planet’s climate sensitivity isn’t nearly as high as previously feared; The Economist reported a common theme: ‘Good news at last?’ . Others were more bullish, with the illustrious Investors.com leading with ‘Global warming models called into question by new study’ and claiming the results showed ‘the scientific fraud at the heart of the anti-global warming movement’. But do the results show there’s nothing to be concerned about?
To best understand the relevance of the latest results on climate sensitivity, it is worth stepping back and reminding ourselves of the context. Most important in this regard is the Intergovernmental Panel on Climate Change (IPCC) 2007 Fourth Assessment Report. This Nobel Prize-winning report summarized the results of numerous computing models made by teams around the world that looked at the likely average temperature of the world at the end of this century. The models covered a number of different scenarios for where concentrations of greenhouse gases in the atmosphere might go in the future. At one extreme, the projections anticipated the world’s economies doing nothing about the problem, pumping ever more gases into the air – wonderfully called ‘business as usual’ – reaching an equivalent carbon dioxide level of 1550 parts per million by 2100.
At the other end of the scale, large cuts are made and gas levels rise at first but then fall back to 600 parts per million. Up until 2030 there isn’t a lot to choose between the models; some parts of the world seem likely to warm more than others, while some appear to cool; overall, they all suggest a warming of 0.6°C relative to 1990. But after 2030, the different scenarios paint widely different pictures. For the low-emissions scenario, temperatures increase on average by 1.8°C by the end of the century. For business as usual, most of the models give a far bleaker view, suggesting an average temperature rise of 4°C. Crucially for these models, the climate sensitivity values used ranged from 2 to 4.5˚C (overlapping with those suggested by Schmittner and colleagues).
Importantly, in a warming world there isn’t just the direct radiative forcing ofgreenhouse gases to consider but also the feedbacks in the system. At present, much of the carbon dioxide released from human activity is being soaked up by the land and oceans; the land appears to be taking up more carbon from a mixture of regrowth on abandoned agricultural land and enhanced photosynthesis brought about by ever increasing atmospheric levels; in the oceans, CO2 is dissolving in the surface waters where it can be fixed by surface-dwelling plants and animals and later pumped to great depth for burial. The risk with future warming is these sinks will become sources of greenhouse gases and return the carbon to the atmosphere, amplifying the warming. Simulations from the Coupled Climate Carbon Cycle Model Intercomparison (C4MIP) looked at the scale of this positive feedback for the end of the century and found an additional warming by as much as 1.5˚C; whether the land or ocean will be the dominant source of extra carbon to the atmosphere remains the big question.
Although the results by Schmittner and colleagues provide a new insight into our planet’s last ice age it is important to remember that this period is not necessarily the best analogue for a future warmer world; after all, an ice age is not where we’re heading. Regardless of where the carbon in the atmosphere comes from, the distribution of ice sheets, vegetation cover and different modes of ocean circulation 20,000 years ago means this period can really only give us a lower estimate on what might happen in the future.
Perversely, articles such as that on Investors.com imply a couple of extra degrees is nothing to worry about. Unfortunately, studies of the most recent warm ‘interglacial’ – a period commonly referred to as the ‘Eemian’ – have shown this to be far from the case. The Eemian experienced a globally averaged temperature of around 2˚C relative to pre-industrial levels and was associated with a global sea level more than 6.6 metres higher than today . It’s hard to imagine anyone considering this a comforting scenario and raises an important question: can stabilising global temperatures at 2˚C be considered ‘safe’ ? Managing carbon levels in the atmosphere remains an urgent issue, no matter how the media sometimes present the scientific results.