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Thursday, February 20, 2014

John Abraham: Accounting for global warming requires looking at the oceans

Separating natural and human influences on the climate a Herculean task

by John Abraham, "Climate Consensus -- The 97%," The Guardian, February 20, 2014

Separating the human and natural influences on the climate is a tough task. On the other hand, because it is exciting, scientists around the world are working on it every day. One of the most active questions scientists are trying to answer right now is, how much excess energy is the Earth gaining? Quantifying this excess energy and where it ends up, often called balancing the Earth's energy budget, is crucial for understanding the future of the planet. 

There are a variety of ways to measure how much extra energy the Earth is absorbing. First, scientists can measure increases in the energy of oceans, melting of ice, and changes in heat; they can use satellites with sensors to make measurements of the net inflow, or they specify all of the inputs from the sun and how it is modulated by pollution and effects of volcanic eruptions and estimate the energy using climate models.

It is clear, from all of these measurement methods that the Earth is gaining heat. But, how is this energy imbalance impacted by greenhouse gases, changes to the sun's output, natural climatic fluctuations such as El Niño, or increases in particulates in the atmosphere that reflect sunlight?
Scientists Kevin TrenberthJohn Fasullo, and Magdalena Balmaseda studied just these questions. While the details are a bit complicated, the essentials are pretty easy to understand. In their paper Earth's Energy Balance, they asked some straightforward questions. Can we monitor the energy imbalance with direct measurements? Can we track where the energy goes?
First, they used a powerful tool called ocean reanalysis to tie together real-world measurements with climate models. Calculations of the net energy balance were compared with satellite measurements and with simulations from computer climate model calculations.
They found that there are significant month-to-month variations in the Earth energy balance associated with weather patterns and longer cycles associated with ocean fluctuations. Additionally, changes to sun output account for up to about 15% of the recent climate change we've seen. They also found that a comparison of Earth energy imbalances from satellites at times differ from estimates obtained from ocean heating, particularly during 2008-2009. The discrepancy is best illustrated by quoting the article, which states,
"Temperatures were below normal globally and January 2008 was the coldest month relative to normal this century. This led to lower outgoing energy but was accompanied by an increase in absorbed energy, as clouds decreased in amount, leaving a pronounced heating of the planet for about a year during 2008-2009. Moreover, as shown above, ocean temperatures for the upper 700 meters from 2005-2008 suggested a substantial slowing of the increase in ocean heat content precisely during the time when satellite estimates depict an increase in planetary energy imbalance, compounding the difficulty. So, where did the heat go?"
Some of the heat has been accounted for by including energy needed to melt global ice, improvements to satellite and ocean measurements, and including deeper parts of the ocean and the arctic regions, particularly the arctic oceans. However, the books are still not balanced.
What do I think? Well first, the fact that ocean temperatures are increasing and sea level is still rising means that the climate is still warming. There has been no halt or pause. My own research in ocean heat content tells me that. Has the heating of oceans slowed? Perhaps – but perhaps not. My view is the accuracy of the measurements, and a recent uptick in ocean heating, particularly for the upper 2,000 meters, will bring us closer to "closing the books" and accounting for all of the energy.
We also have to learn more about what the very deep ocean is doing – we know heat is going there. Of particular importance is the lack of reliable information on Arctic Ocean warming. Finally, I think that volcanic and human-emitted particulates that have risen since about 2000 are reflecting more sunlight than previously thought. Considering these effects helps in the overall accounting and will be the subject of a future post.
This complex topic clearly involves many different kinds of experts: satellite sensing, radiation, atmospheric, aerosols, clouds, oceans, sea ice, glaciers, climate modeling… and then special generalists to tie everything together and nudge the topic experts to do even better.

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