Cambio climatico

COMMENTARY

Quantifying climate change — too rosy a picture?
STEPHEN E. SCHWARTZ1, ROBERT J. CHARLSON2 AND HENNING RODHE3
The latest report from the Intergovernmental Panel on Climate Change assesses the skill of climate models by their ability to reproduce warming over the twentieth century, but in doing so may give a false sense of their predictive capability.

he challenge of climatechange research is to develop confident predictive capability. Given prospective future emissions of greenhouse gases and other climate influencing substances such as aerosols, what changes in global mean surface temperature and other climate attributes can be expected, and what confidence can be placed in these projected changes? The single most important concept here is the relationship betweenclimate forcing and the response of the system. Forcing (measured in watts per square metre) is the global mean change in energy balance imposed over time by changes in atmospheric composition (for example CO2, CH4 and aerosols) and other influences such as land use. It is the key diagnostic of human climate perturbation.

Anthropogenic

T

RF terms

Radiative forcing components CO2

RFvalues (W m–2) Spatial scale LOSU 1.66 [1.49 to 1.83] 0.48 [0.43 to 0.53] 0.16 [0.14 to 0.18] 0.34 [0.31 to 0.37] Global High

Long-lived greenhouse gases

N2O CH4 Stratospheric Halocarbons

Global

High

Ozone Stratospheric water vapour from CH4 Surface albedo Direct effect Total aerosol Cloud albedo effect

Tropospheric

–0.05 [–0.15 to 0.05] Continental to global 0.35 [0.25 to 0.65]Med Low Med – low Med – low Low Low Low

0.07 [0.02 to 0.12] Land use Black carbon on snow –0.2 [–0.4 to 0.0] 0.1 [0.0 to 0.2] –0.5 [–0.9 to 0.1]

Global Local to continental Continental to global

Continental –0.7 [–1.8 to –0.3] to global 0.01 [0.003 to 0.03] Continental 0.12 [0.06 to 0.30] Global

Linear contrails Natural

Solar irradiance Total net anthropogenic

1.6 [0.6 to 2.4]6.7 [4.2 to 9.2]

2100 –2 –1 0 Radiative forcing (W m–2) 1 2 3 4 5 6 7

8

9

10

NEW DIRECTIONS

In its latest report on the physical science basis of climate change1, the Intergovernmental Panel on Climate Change (IPCC) moves increasingly beyond detecting global change and attributing it to human activity, into quantitative assessment of the ability to forecast the prospectivechange in climate that would result from future emissions scenarios. This new direction requires careful analysis of the uncertainties associated with assessing future climate change, and the new report is explicit in its definitions. In a departure from previous reports, the latest assessment gives a best estimate of climate sensitivity — the increase in global mean surface temperature that would beexpected to result from a doubling of atmospheric CO2 levels. The present best estimate

Figure 1 Global average radiative forcing (RF) estimates and uncertainty ranges in 2005, relative to the preindustrial climate, for anthropogenic carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) ofthe forcing and the assessed level of scientific understanding (LOSU). The total anthropogenic radiative forcing and its associated uncertainty (5–95% confidence interval) are also shown1. Added to the figure (green bar at bottom and associated uncertainty range) is the estimate from the 2001 IPCC report2 of the total forcing projected for 2100, where the uncertainty denotes the range ofestimates for different emission scenarios.

of 3 °C is the same as that given by a US National Research Council panel in 1979, but it now comes with a welldefined confidence interval. Specifically, the report states that the sensitivity “is likely to be in the range 2 °C to 4.5 °C,” where “likely” is defined to mean a greater than 66% probability that the actual quantity is within the stated range....