Black Carbon's Day on the Hill


















 Drew Shindell testifies as Veerabhadran Ramanathan looks on.
Credit: Committee on Energy Independence and Global Warming


Adam Voiland, NASA’s Earth Science News Team

Have the Last Four Summers and Winters Felt Warmer?













–Adam Voiland, NASA’s Earth Science News Team

Are the Oceans Really Stuffed to the Gills with Carbon Dioxide?

Two months ago, NASA’s Timothy Hall and colleagues published a study that described how they had estimated the amount of manmade carbon dioxide absorbed by the ocean since the start of the industrial era.

Oceans absorb about a third of the carbon dioxide that humans release into the atmosphere, so sorting out a long-term record of carbon uptake is of great interest to climate scientists.

To create their record of the ocean’s uptake of carbon, Hall and Samar Khatiwala, the lead author of the study, devised a clever mathematical technique that proved to be a considerable advance. When Hall’s study appeared in the journal Nature, he assumed the creation of this new long-term, continuous record would headline the news.

But journalists gravitated toward something else entirely: a brief mention that the amount of carbon dioxide absorbed by the ocean seemed to be experiencing, as the researchers put it, “a small decline in the rate of increase in the last few decades.”

“Seas Grow Less Effective at Absorbing Emissions”, one headline trumpeted. Another article compared the world’s oceans to a fish “stuffed to the gills” with carbon dioxide and another reported a “sudden and dramatic drop in the amount of carbon dioxide being absorbed by the sea.

Given the caveats included in the original study, all of this caught Hall slightly off guard. I’ll let Hall, who summarized his reactions to the coverage for What On Earth, pick the story up from here:

My coauthors and I had viewed the ability to estimate the history of ocean uptake of anthropogenic carbon as the highlight of the paper. Previously, observationally-based estimates had only provided a few snapshots in time, and we were proud of the cleverness of our techniques.

It seems clever mathematical techniques, however, don’t make good press releases. Interestingly, coverage of the paper has not focused on the fact that we can estimate the uptake history. Instead it has focused on apparent reductions in the rate of uptake over the last 2 decades.

The figure below shows our estimate of ocean uptake since 1775. The first impression is the rapid increase since 1950, coinciding with the rapid rise in carbon emissions to the atmosphere. The oceans have prevented about 1/3 of anthropogenic carbon emissions from accumulating in the atmosphere. A closer reading of the curve reveals a reduction in the uptake’s rate of increase after about 1980, even while emissions continue to increase.

Scientists have long suspected that ocean carbon uptake would eventually be unable to keep pace with rising emissions. Basic aqueous chemistry tells us that, as dissolved carbon in seawater increases, seawater becomes less able to absorb new carbon. Eventually, the absorption saturates. The slowing down of the increase rate may be an early signal of this saturation.

However, recent changes in uptake were not our focus when we performed the study, and more importantly we did not analyze the statistical significance of the slowdown. We plan further analysis on these trend variations. What we can say is that there are physical reasons to suspect a reduction in the ocean’s capacity to keep pace with increasing carbon emissions, and that there are now strong observational hints for recent reductions.

Hall advises reading this story, which also appeared in Nature. It’s less dramatic and more technical than most of media accounts, but it is a more accurate representation of the paper.

–Adam Voiland, NASA’s Earth Science News Team
   Image Credit: (EPOD/K. Chrisodoulopoulus)

How Do Global Soot Models Measure Up?

A image from a simulation that shows the spread of black carbon aerosols in Asia. Areas where the air was thick with
the pollution particles are white, while lower concentrations are transparent purple. (Credit:
Earth Observatory)

As NASA atmospheric scientist Eric Wilcox recently told Time magazine, emerging evidence suggests that a short-lived type of air pollution called black carbon—known popularly as soot—can exacerbate global warming by absorbing incoming solar radiation.

Yet pinning down precisely how much the black carbon exacerbates warming is no easy task, research conducted by Goddard Institute for Space Studies climatologist Dorothy Koch suggests. The study, published in Atmospheric Chemistry and Physics tracked how the predictions from 17 global black carbon models compared with actual measurements collected by airplane, satellite, and ground-based sensors. It shows, among other things, that models generally underestimate black carbon’s warming effect on climate.

Koch tested all the models in three ways. In the simplest of the three, she compared the models’ predictions to the amount of black carbon measured at the surface, finding that they matched real life reasonably well.

Her second test compared the models’ predictions to black carbon measurements made higher in the atmosphere using airplanes, and the results were much less clear cut. Though the models usually had too much black carbon over pollution sources, most had too little over remote regions such as the Arctic.

Koch’s final and most important test looked at how much solar radiation black carbon actually absorbs, an indicator of the amount of warming the particles actually produce. Again, the results were mixed. The models were largely accurate over North America and Europe, but were not for areas that have high levels of black carbon such as Central Africa, Southeast Asia, and the Amazon.

In a write-up on the Goddard Institute for Space Studies web site, Koch summarizes her findings this way:

We concluded from this study that most models have enough black carbon at ground level in polluted regions, too much in the atmosphere above source regions, but not enough in the Arctic where black carbon may play an important role in contributing to Arctic warming and ice/snow melt. The models’ soot generally does not absorb enough sunlight and therefore these models would underestimate black carbon heating effects. This probably results from underestimating the absorbing properties of the particles rather than the amount (mass) of black carbon.

Wondering how climate modelers can continue to close the gap between model predictions and reality? Koch put forward some advice on how to fine-tune the next generation of aerosols models. Her top three:

1) Account for mixing between black carbon and other components of the atmosphere,
2) Incorporate better measurements of particle size and source amount in some regions.
3) Continue to mine ongoing satellite and field campaigns for data about black carbon.

You can read more GISS science briefs and NASA news stories about black carbon here, here, and here.

–Adam Voiland, NASA’s Earth Science News Team