The Earth has warmed substantially less than would have been expected during the industrial era based on current best estimates of Earth’s “climate sensitivity” – the amount of global temperature rise predicted in response to a given rise in atmospheric concentrations of carbon dioxide (CO2).
A study just published in the Journal of Climate, Stephen Schwartz, of Brookhaven National Laboratory, and colleagues examines the reasons for this discrepancy.
According to frequently used estimates of climate sensitivity there should be a global temperature rise of 2 deg C which is greater than the 0.8 deg C increase observed (which prior to 1950 also contains a solar warming component.) Schwartz’s analysis attributes the reasons for the discrepancy to a possible mix of two major factors: 1) Earth’s climate may be less sensitive to rising greenhouse gases than currently assumed and/or 2) reflection of sunlight by haze particles – aerosols – in the atmosphere may be offsetting some of the expected warming.
“Because of present uncertainties in climate sensitivity and the enhanced reflectivity of haze particles,” said Schwartz, “it is impossible to accurately assign weights to the relative contributions of these two factors. This has major implications for understanding of Earth’s climate and how the world will meet its future energy needs.”
A key question facing policymakers is how much additional CO2 and other heat-trapping gases can be introduced into the atmosphere, beyond what is already present, without dangerous warming. Many consider the threshold for dangerous climatic effects to be 2 deg C above the preindustrial level, although there are considerable uncertainties.
The paper in the Journal of Climate suggests that if the Earth’s climate sensitivity is at the low end of current estimates as given by the Intergovernmental Panel on Climate Change, then the total maximum future emissions of heat-trapping gases so as not to exceed the 2 deg C would correspond to about 35 years of present annual emissions of CO2. A climate sensitivity at the present best estimate would mean that no more heat-trapping gases can be added.
This paper is another example that IPCC estimates of future temperature trends have limited scientific value and need re-evaluating.
Schwartz observes that formulating energy policy with the present uncertainty in climate sensitivity is like navigating a large ship in perilous waters without charts. “We know we have to change the course of this ship, and we know the direction of the change, but we don’t know how much we need to change the course or how soon we have to do it.”