Science, Vol 308, Issue 5727, 1431-1435, 3 June 2005
Earth's Energy Imbalance: Confirmation and Implications
James Hansen,1,2* Larissa Nazarenko,1,2 Reto Ruedy,3 Makiko Sato,1,2 Josh Willis,4 Anthony Del Genio,1,5 Dorothy Koch,1,2 Andrew Lacis,1,5 Ken Lo,3 Surabi Menon,6 Tica Novakov,6 Judith Perlwitz,1,2 Gary Russell,1 Gavin A. Schmidt,1,2 Nicholas Tausnev3
Our climate model, driven mainly by increasing human-made greenhouse gases and aerosols, among other forcings, calculates that Earth is now absorbing 0.85 ± 0.15 watts per square meter more energy from the Sun than it is emitting to space. This imbalance is confirmed by precise measurements of increasing ocean heat content over the past 10 years. Implications include (i) the expectation of additional global warming of about 0.6°C without further change of atmospheric composition; (ii) the confirmation of the climate system's lag in responding to forcings, implying the need for anticipatory actions to avoid any specified level of climate change; and (iii) the likelihood of acceleration of ice sheet disintegration and sea level rise.
1 NASA Goddard Institute for Space Studies, New York, NY 10025, USA.
2 Columbia Earth Institute, Columbia University, New York, NY 10025, USA.
3 SGT Incorporated, New York, NY 10025, USA.
4 Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
5 Department of Earth and Environmental Sciences, Columbia University, New York, NY 10025, USA.
6 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
* To whom correspondence should be addressed. E-mail: firstname.lastname@example.org
Earth's climate system has considerable thermal inertia. This point is of critical importance to policy- and decision-makers who seek to mitigate the effects of undesirable anthropogenic climate change. The effect of the inertia is to delay Earth's response to climate forcings, i.e., changes of the planet's energy balance that tend to alter global temperature. This delay provides an opportunity to reduce the magnitude of anthropogenic climate change before it is fully realized, if appropriate action is taken. On *the other hand, if we wait for more overwhelming empirical evidence of climate change, the inertia implies that still greater climate change will be in store, which may be difficult or impossible to avoid.
The primary symptom of Earth's thermal inertia, in the presence of an increasing climate forcing, is an imbalance between the energy absorbed and emitted by the planet. This imbalance provides an invaluable measure of the net climate forcing acting on Earth. Improved ocean temperature measurements in the past decade, along with high-precision satellite altimetry measurements of the ocean surface, permit an indirect but precise quantification of Earth's energy imbalance. We compare observed ocean heat storage with simulations of global climate change driven by estimated climate forcings, thus obtaining a check on the climate model's ability to simulate the planetary energy imbalance.
The lag in the climate response to a forcing is a sensitive function of equilibrium climate sensitivity, varying approximately as the square of the sensitivity (1), and it depends on the rate of heat exchange between the ocean's surface mixed layer and the deeper ocean (24). The lag could be as short as a decade, if climate sensitivity is as small as 0.25°C per W/m2 of forcing, but it is a century or longer if climate sensitivity is 1°C per W/m2 or larger (1, 3). Evidence from Earth's history (36) and climate models (7) suggests that climate sensitivity is 0.75° ± 0.25°C per W/m2, implying that 25 to 50 years are needed for Earth's surface temperature to reach 60% of its equilibrium response (1).
We investigate Earth's energy balance via computations with the current global climate model of the NASA Goddard Institute for Space Studies (GISS). The model and its simulated climatology have been documented (8), as has its response to a wide variety of climate forcing mechanisms (9). The climate model's equilibrium sensitivity to doubled CO2 is 2.7°C (
Climate forcings. Figure 1A summarizes the forcings that drive the simulated 1880 to 2003 climate change. Among alternative definitions of climate forcing (9), we use the effective forcing, Fe. Fe differs from conventional climate forcing definitions (11) by accounting for the fact that some forcing mechanisms have a lesser or greater "efficacy" in altering global temperature than an equal forcing by CO2 (9). Fe is an energy flux change arising in response to an imposed forcing agent. It is constant throughout the atmosphere, because it is evaluated after atmospheric temperature has been allowed to adjust to the presence of the forcing agent.
The largest forcing is due to well-mixed greenhouse gases (GHGs)CO2, CH4, N2O, CFCs (chlorofluorocarbons)and other trace gases, totaling 2.75 W/m2 in 2003 relative to the 1880 value (Table 1). Ozone (O3) and stratospheric H2O from oxidation of increasing CH4 bring the total GHG forcing to 3.05 W/m2 (9). Estimated uncertainty in the total GHG forcing is