An Unmeasured Forcing

“It is remarkable and untenable that the second largest forcing
that drives global climate change remains unmeasured,” writes Dr. James Hansen, the head of NASA’s climate change research team, and arguably the world’s top climatologist.

The word “forcing” refers to a factor, such as changes in the Sun’s output or in atmospheric composition, that exerts a warming or cooling influence on the Earth’s climate. The climate doesn’t magically change for no reason – it is always driven by something. Scientists measure these forcings in Watts per square metre – imagine a Christmas tree lightbulb over every square metre of the Earth’s surface, and you have 1 W/m2 of positive forcing.

Currently, the largest forcing on the Earth’s climate is that of increasing greenhouse gases from burning fossil fuels. These exert a positive, or warming, forcing, hence the term “global warming”. However, a portion of this positive forcing is being cancelled out by the second-largest forcing, which is also anthropogenic. Many forms of air pollution, collectively known as aerosols, exert a negative (cooling) forcing on the Earth’s climate. They do this in two ways: the direct albedo effect (scattering solar radiation so it never reaches the planet), and the indirect albedo effect (providing surfaces for clouds to form and scatter radiation by themselves). A large positive forcing and a medium negative forcing sums out to a moderate increase in global temperatures.

Unfortunately, a catch-22 exists with aerosols. As many aerosols are directly harmful to human health, the world is beginning to regulate them through legislation such as the American Clean Air Act. As this pollution decreases, its detrimental health effects will lessen, but so will its ability to partially cancel out global warming.

The problem is that we don’t know how much warming the aerosols are cancelling – that is, we don’t know the magnitude of the forcing. So, if all air pollution ceased tomorrow, the world could experience a small jump in net forcing, or a large jump. Global warming would suddenly become much worse, but we don’t know just how much.

The forcing from greenhouse gases is known with a high degree of accuracy – it’s just under 3 W/m2. However, all we know about aerosol forcing is that it’s somewhere around -1 or -2 W/m2 – an estimate is the best we can do. The reason for this dichotomy lies in the ease of measurement. Greenhouse gases last a long time (on the order of centuries) in the atmosphere, and mix through the air, moving towards a uniform concentration. An air sample from a remote area of the world, such as Antarctica or parts of Hawaii, will be uncontaminated by cars and factories nearby, and will contain an accurate value of the global atmospheric carbon dioxide concentration (the same can be done for other greenhouse gases, such as methane) . From these measurements, molecular physics can tell us how large the forcing is. Direct records of carbon dioxide concentrations have been kept since the late 1950s:

However, aerosols only stay in the troposphere for a few days, as precipitation washes them out of the air. For this reason, they don’t have time to disperse evenly, and measurements are not so simple. The only way to gain accurate measurements of their concentrations is with a satellite. NASA recently launched the Glory satellite for just this purpose. Unfortunately, it failed to reach orbit (an inherent risk for satellites), and given the current political climate in the United States, it seems overly optimistic to hope for funding for a new one any time soon. Luckily, if this project was carried out by the private sector, without the need for money-draining government review panels, James Hansen estimates that it could be achieved with a budget of around $100 million.

An accurate value for aerosol forcing can only be achieved with accurate measurements of aerosol concentration. Knowing this forcing would be immensely helpful for climate researchers, as it impacts not only the amount of warming we can expect, but also how long it will take to play out, until the planet reaches thermal equilibrium. Aimed with better knowledge of these details will allow policymakers to better plan for the future, regarding both mitigation of and adaptation to climate change. Finally measuring the impact of aerosols, instead of just estimating, could give our understanding of the climate system the biggest bang for its buck.


7 thoughts on “An Unmeasured Forcing

  1. Thanks for the informative post. I try to follow all Earth changes in order to put the pieces together from a non-scientific perspective. Climate change is happening as it has in the past, I just hope we’re ready to adapt to it.

  2. [The Glory satellite failed to reach orbit] “… if this project was carried out by the private sector…”

    Of course, that’s an absolutely huge if, and underscores how badly our civilisation is organised. Naturally the private sector won’t invest in such projects as there’s no profit in launching research satellites. No doubt it would leap at the chance if there was a quick buck to be made.

    Money — the root of all stupidity.

  3. A very clear and informative post. This was the type of post I wish I had read way back when I was first trying to understand the issues. People kept using the word “forcings” but didn’t define it so I was unsure what was meant or what it might be referring to. At the time, there were no explanatory websites (least none that I could find–all the good ones have shown up since then), so readers new to the topic were jumping into the deep end of the pool.

  4. In James Hansen’s recently published book I recall him specifically mentioning the question of aerosols, while mentioning that there existed no satellite which gathered data for the pupose or that could be used for the purpose. When reading about the failure of the Glory satellite I felt disapointed because I suspected that a replacement would not be put in place in a timely manner. Are we to hope that Japan, or the ESA, or India, or China will act in the place of NASA? It seems that the talk in the United States of the “need for further study,” in the sense of expanding in a significant way the size of the effort being made is a deceit, because it seems to me that an expansion of effort would me a redundancy of effort, such that set backs of this sort would have little material consequence.

    What, you might ask, have you just realized this? No. However, I would rather not have the notion confirmed.


  5. Kate

    Good job covering the topic.

    Craig & Joe Ardolino

    “Climate change is happening as it has in the past, I just hope we’re ready to adapt to it.”

    Climate has certainly changed in the past, but what is happening now has never occurred before. Never before has a species (humans) taken the carbon, that nature had sequestered over something like 100 million years, and returned it to the active carbon cycle. You may have heard of “clean coal” technology called carbon capture and sequestration. Nature has been doing that for all those millions of years. It’s called coal and oil.

    Now humans are putting all that carbon back into the atmosphere and active carbon cycle in a few hundred years, the blink of an eye in geological time.

    The active or short term carbon cycle is the processes by which carbon cycles through the atmosphere, water, topsoils and all living things over relatively short time scales, compared with the millions and millions of years of geological time scales. This short term carbon cycle has been in a kind of balance for far longer than human civilization or maybe even the human species has been around.
    It’s critical to the only world we know. One thing the skeptics have right. Carbon is indeed an essential element, without which we wouldn’t exist. The most important element in our world, would probably not be a stretch. And that is exactly why it is important not to mess up the carbon cycle.

    Another way we may be messing up the carbon cycle is by ocean acidification.
    Tiny plankton, called coccolithophores, are armored with calcium carbonate shells, just like the more familiar shellfish. Excess CO2 from human emissions is dissolving in seawater, forming carbonic acid. The calcium carbonate needed for shell formation is base, opposite of acid. If ocean acidification threatens these plankton, we could lose a large carbon sink. When they die, their shells fall to the deep sea floor, locking them away in the long term carbon cycle of geological time scales, and out of the short term carbon cycle. Not to mention that these plankton are at the bottom of the food chain.

    Adaptation? Yes, but CO2 mitigation is where most of our efforts should be made. Otherwise, it might become something we can’t adapt to. The best results will come from front end loading the effort. Do as much as possible early.

    If you like science, I recommend the book “The Carbon Age” by Eric Roston.
    Written for non experts.

  6. It is correct that we are not able to measure aerosols completely. However we have enough empirical data to make correlations and relationships into the aerosol effect.

    Aerosols are more then the stuff that come out of spray cans, they are all the particulate matter that (human made or is naturally occurring) that goes into the atmosphere. The most notable of these was during 1940’s through the 1970’s when temperature actually decreased but CO2 emissions were increasing. Scientist now know that, the decrease was due to aerosols. At the time coal power plants were operating without scrubbers. In the 1970s with the addition of scrubbers their effects were negated.

    So from those correlations, NASA was able to update their climate models with aerosols in mind.

    1991 the eruptions of mount Pinatubo ejected over 20 million tons of aerosols and this caused a global temperature decrease of 0.3C. At the time the model produced by NASA (of which Hasen was lead scientist) accurately predicted the temperature drop.

    But of course it would be good to know exactly how much aerosols are in the atmosphere. I hope that a new satellite will be launched post haste.

    source: “The Weather Makers” by Tim Flannery, pg. 157-160, 162

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