Earth's Energy Imbalance and Implications

James Hansen, Makiko Sato, Pushker Kharecha, Karina von Schuckmann

Improving observations of ocean temperature confirm that Earth is absorbing more energy from the sun than it is radiating to space as heat, even during the recent solar minimum. This energy imbalance provides fundamental verification of the dominant role of the human-made greenhouse effect in driving global climate change. Observed surface temperature change and ocean heat gain constrain the net climate forcing and ocean mixing rates. 

 We conclude that most climate models mix heat too efficiently into the deep ocean and as a result underestimate the negative forcing by human-made aerosols. Aerosol climate forcing today is inferred to be ‒1.6 ± 0.3 W/m2, implying substantial aerosol indirect climate forcing via cloud changes. Continued failure to quantify the specific origins of this large forcing is untenable, as knowledge of changing aerosol effects is needed to understand future climate change. 

A recent decrease in ocean heat uptake was caused by a delayed rebound effect from Mount Pinatubo aerosols and a deep prolonged solar minimum. Observed sea level rise during the Argo float era can readily be accounted for by thermal expansion of the ocean and ice melt, but the ascendancy of ice melt leads us to anticipate a near-term acceleration in the rate of sea level rise.

Humanity is potentially vulnerable to global temperature change, as discussed in the Intergovernmental Panel on Climate Change (IPCC, 2001, 2007) reports and by innumerable authors. Although climate change is driven by many climate forcing agents and the climate system also exhibits unforced (chaotic) variability, it is now widely agreed that the strong global warming trend of recent decades is caused predominantly by human-made changes of atmospheric composition (IPCC, 2007).

The basic physics underlying this global warming, the greenhouse effect, is simple. An increase of gases such as CO2 makes the atmosphere more opaque at infrared wavelengths. This added opacity causes the planet's heat radiation to space to arise from higher, colder levels in the atmosphere, thus reducing emission of heat energy to space. The temporary imbalance between the energy absorbed from the sun and heat emission to space, causes the planet to warm until planetary energy balance is restored.

The planetary energy imbalance caused by a change of atmospheric composition defines a climate forcing. Climate sensitivity, the eventual global temperature change per unit forcing, is known with good accuracy from Earth's paleoclimate history. However, two fundamental uncertainties limit our ability to predict global temperature change on decadal time scales.

First, although climate forcing by human-made greenhouse gases (GHGs) is known accurately, climate forcing caused by changing human-made aerosols is practically unmeasured. Aerosols are fine particles suspended in the air, such as dust, sulfates, and black soot (Ramanathan et al., 2001). Aerosol climate forcing is complex, because aerosols both reflect solar radiation to space (a cooling effect) and absorb solar radiation (a warming effect). In

Fig. 1.  Climate forcings employed in this paper.  Forcings through 2003 (vertical line) are the same as used by Hansen et al. (2007b), except the aerosol forcing after 1990 is approximated as -0.5 times the GHG forcing.  Aerosol forcing includes all aerosol effects, including indirect effects on clouds and snow albedo.  GHGs include O3 and stratospheric H2O, in addition to well-mixed GHGs.
addition, atmospheric aerosols can alter cloud cover and cloud properties.  Therefore, precise  composition-specific measurements of aerosols and their effects on clouds are needed to assess the aerosol role in climate change.

Second, the rate at which Earth's surface temperature approaches a new equilibrium in  response to a climate forcing depends on how efficiently heat perturbations are mixed into the deeper ocean.  Ocean mixing is complex and not necessarily simulated well by climate models.  Empirical data on ocean heat uptake are improving rapidly, but still suffer limitations.

We summarize current understanding of this basic physics of global warming and note observations needed to narrow uncertainties.  Appropriate measurements can quantify the major factors driving climate change, reveal how much additional global warming is already in the pipeline, and help define the reduction of climate forcing needed to stabilize climate.  Continued...

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