As the world awaits the Intergovernmental Panel on Climate Change's (IPCC) latest verdict on the state of the climate, new research out this year finds that climate change could have double the impact previously thought.
The peer-reviewed study published in the Quarterly Journal of the Royal Meteorological Societyargues that conventional conclusions on climate sensitivity - the extent to which global temperatures respond to greenhouse gas emissions - underestimate the role of some amplifying feedbacks that may intensify climate impacts in ways that many models tend to overlook.
Traditional estimates of climate sensitivity such as that adopted by the IPCC focus on "fast feedbacks" like water vapour, natural aerosols, clouds, and snow cover, but do not sufficiently account for slower feedbacks including "surface albedo feedbacks from changes in continental ice sheets and vegetation", and climate greenhouse gas feedbacks "from changes in natural (land and ocean) carbon sinks."
These types of feedbacks refer to self-reinforcing process which, once human-induced emissions create a change in a particular eco-system, lead to further changes beyond the initial human forcing as different parts of the system continue to respond. With 'albedo', for instance, the reduction of snow and ice cover due to melting induced by global warming means less surfaces reflecting sunlight back into the atmosphere, and thus more absorption of heat, which leads to further melting - and potentially a self-reinforcing cycle that contributes further to overall warming.
With 'carbon sinks', as the oceans absorb CO2 and excess heat due to global warming, they could reach a saturation point where their ability to absorb is continually reduced, in turn allowing global warming to accelerate - eventually, the oceans themselves could become an increasing source of CO2 if this process continues.
Climate sensitivity estimates based on fast feedbacks alone, ignoring the above processes, average out at suggesting a doubling of carbon dioxide (CO2) emissions would lead to a global temperature rise of about 3C. However, the new paper by a multidisciplinary team led by Columbia University's Earth Institute, notes that ice sheet and vegetation surface have wrongly been assumed to be irrelevant "based on the long-standing notion that continental ice sheet changes occur so slowly (over several millennia)."
The paper cites "evidence from the palaeoclimatic record for sea-level changes of several metres per century" as well as "present-day observations of increasing melt and overall mass loss from Greenland and Antarctica", which together "imply that ice sheet changes can occur more rapidly than previously recognized." They also point to several studies indicating that "significant vegetation response can occur on decadal-to centennial time-scales." Taking these processes into account gives an estimate known as the 'Earth system sensitivity', which the study finds is double that of other estimates at between 6 to 8C. The dramatic changes that this higher sensitivity implies would occur over "several centuries to about a millennium", if not "several millennia."
Despite that long time-scale, unfortunately some early impacts could still be seen this century. The study warns:
"The higher Earth system sensitivity thus implies a real possibility of exceeding the 2C global warming threshold if atmospheric GHG concentrations are sustained at or above present-day levels. This needs to be communicated clearly to policymakers and to the general public in order to ensure appropriately informed decisions about future GHG stabilization."
The difficulties in estimating the Earth system sensitivity, the paper points out, are due to "the lack of palaeo-analogues for the present-day anthropogenic forcing" as well as because "current models are unable to adequately simulate the physics of ice sheet decay and certain aspects of the natural carbon and nitrogen cycles." More