http://www.theguardian.com/environment/earth-insight/2013/sep/18/climate-change-double-impact-study

 

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 Society argues 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."

The new study shows that far from the IPCC's models being too alarmist, they are in fact too conservative. Lead author Michael Previdi, assistant research professor at Lamont-Doherty Earth Observatory at Columbia University's Earth Institute, said:

"Traditionally, such as in the assessment reports of the IPCC, only feedbacks from changes in water vapour, clouds and sea ice were considered in the definition of climate sensitivity. However, the inclusion of feedbacks due to changes in the natural land and ocean carbon sinks has the advantage of more directly linking man-made greenhouse gas emissions with the ensuing global temperature increase. This provides a truer indication of the climate sensitivity to human perturbations."

Although many impacts will take centuries if not longer to play out, according to Previdi the study's findings imply that a business-as-usual trajectory could have more serious implications than hitherto recognised, within this century alone.

"Some of the additional feedbacks that we consider in our framework, in particular feedbacks due to changes in the natural land and ocean carbon sinks, could begin to come into play during this century", said Previdi. "This means that the Earth system is more sensitive to a business-as-usual scenario than previously thought."

This in turn means that staying below the 2C warming target accepted as the safe limit by policymakers will require far greater emissions reductions than are currently on the table. Previdi said:

"The Earth system climate sensitivity framework we suggest implies the need to reduce future man-made greenhouse gas emissions even further than previously thought in order to avoid the suggested 2C global warming target."

One early study which attempted to model the potential impact of amplifying feedbacks by scientists at Lawrence Berkeley National Laboratory and the University of California, Berkeley, found that the IPCC's worst case scenario for fossil fuel emissions projecting a rise of 6C by 2100 could be too low. Inclusion of "positive feedback loops" suggests that global average temperatures could reach as high as 8C by then. Although uncertainties remain meaning that actual temperatures might be lower even on a worst case scenario, the authors of that paper warned that their research proved "a higher risk that we will experience more severe, not less severe, climate change than is currently forecast."