Hansen: Earth’s Energy Imbalance

“The measured imbalance confirms that, if other climate forcings are fixed, atmospheric CO2 must be reduced to about 350 ppm or less to stop global warming.”

NASA’s James Hansen writes:

Deployment of an international array of Argo floats, measuring ocean heat content to a depth of 2000 m, was completed during the past decade, allowing the best assessment so far of Earth’s energy imbalance. The observed planetary energy gain during the recent strong solar minimum reveals that the solar forcing of climate, although significant, is overwhelmed by a much larger net human-made climate forcing. The measured imbalance confirms that, if other climate forcings are fixed, atmospheric CO2 must be reduced to about 350 ppm or less to stop global warming. In our recently published paper (Hansen et al., 2011), we also show that climate forcing by human-made aerosols (fine particles in the air) is larger than usually assumed, implying an urgent need for accurate global aerosol measurements to help interpret continuing climate change.

Figure 1. Contributions to Earth’s (positive) energy imbalance in 2005-2010. Estimates for the deep Southern and Abyssal Oceans are by Purkey and Johnson (2010) based on sparse observations. (Credit: NASA/GISS)
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Earth’s energy imbalance is the difference between the amount of solar energy absorbed by Earth and the amount of energy the planet radiates to space as heat. If the imbalance is positive, more energy coming in than going out, we can expect Earth to become warmer in the future — but cooler if the imbalance is negative. Earth’s energy imbalance is thus the single most crucial measure of the status of Earth’s climate and it defines expectations for future climate change.

Energy imbalance arises because of changes of the climate forcings acting on the planet in combination with the planet’s thermal inertia. For example, if the Sun becomes brighter, that is a positive forcing that will cause warming. If Earth were like Mercury, a body composed of low conductivity material and without oceans, its surface temperature would rise quickly to a level at which the planet was again radiating as much heat energy to space as the absorbed solar energy.

Earth’s temperature does not adjust as fast as Mercury’s due to the ocean’s thermal inertia, which is substantial because the ocean is mixed to considerable depths by winds and convection. Thus it requires centuries for Earth’s surface temperature to respond fully to a climate forcing…

Read Hansen’s entire article.