One important piece of evidence came from Jo House, from the University of Bristol, and her colleagues, who have modelled the feedback effect that climate change will have on carbon cycles. For the first time, House and the team have shown what kind of effect these feedbacks could have on various different greenhouse-emission target plans, including the G8 targets and the Stern Review.
The researchers modelled how future climate change may alter the way that carbon dioxide moves between the oceans, atmosphere and land – the carbon cycle.
As ocean waters become warmer, the solubility of carbon dioxide in water decreases, meaning that oceans draw down less carbon dioxide from the atmosphere. What’s more, global warming is likely to change the structure of the ocean. “When the water at the top of the ocean warms it makes it harder to mix with the layers below. The ocean tends to stratify,” House told environmentalresearchweb. This lack of mixing will make the upper layer carbon rich and less able to suck down carbon dioxide from the atmosphere.
On land, warmer temperatures speed up the rate of decay of plants, releasing carbon dioxide back into the atmosphere at a faster rate than during cooler times. In addition, rainfall patterns change and previously lush areas may be stricken by drought. “If large areas of forest, such as parts of the Amazon, die back then there won’t be as many plants to suck down carbon dioxide from the atmosphere,” explains House.
Taking these feedback loops into account, House and colleagues have modelled how global-temperature and carbon-dioxide levels will respond under various emissions scenarios.
The G8 plan aims to cut global emissions by 50% by 2050, and then hold emissions constant. Writing in the journal Environmental Research Letters, House and her colleagues show that under this scenario atmospheric carbon-dioxide concentrations will continue to rise despite the cut in emissions. The model suggests that by 2100, carbon-dioxide concentrations could be as high as 590 parts per million (ppm) and global temperatures could be as much as 3.1°C above pre-industrial levels. And by 2300, the worst-case scenario shows that carbon-dioxide levels could be 980 ppm with an accompanying rise in global temperature of 5.7°C.
Things look a little more hopeful if the Stern Review proposal is followed; of cutting emissions by 25% by 2050, rising to 80% by around 2150. In this case, House and colleague’s model showed that carbon-dioxide levels would nearly stabilize at levels of between 500–600 ppm by 2100. However levels would creep up further into the future without greater cuts.
Long term stabilization of atmospheric carbon dioxide at levels of around 550 ppm will require cuts in emissions of between 81 and 90% by 2300, and even more beyond that time, according to House.
It is no good making cuts into another greenhouse gas, such as methane, while ignoring carbon dioxide. “Emitted carbon dioxide persists for centuries to millennia, and reductions of other greenhouse gases cannot compensate for the long-term effects of emitting carbon dioxide,” says House.
Tackling the problem of global warming seems even more daunting when climate-change feedbacks are taken into account, but House doesn’t think we should feel despondent and give up. “It should encourage us to carry on making the small cuts in emissions that we are making, because whatever you do will have a beneficial long-term legacy,” she says.