Modelling Geoengineering

Later in my career as a climate modeller, I expect to spend a lot of time studying geoengineering. Given the near-total absence of policy responses to prevent climate change, I think it’s very likely that governments will soon start thinking seriously about ways to artificially cool the planet. Who will they come to for advice? The climate modellers.

Some scientists are pre-emptively recognizing this need for knowledge, and beginning to run simulations of geoengineering. In fact, there’s an entire model intercomparison project dedicated to this area of study. There’s only a small handful of publications so far, but the results are incredibly interesting. Here I summarize two recent papers that model solar radiation management: the practice of offsetting global warming by partially blocking sunlight, whether by seeding clouds, adding sulfate aerosols to the stratosphere, or placing giant mirrors in space. As an added bonus, both of these papers are open access.

A group of scientists from Europe ran the same experiment on four of the world’s most complex climate models. The simulation involved instantaneously quadrupling CO2 from preindustrial levels, but offsetting it with a reduction in the solar constant, such that the net forcing was close to zero.

The global mean temperature remained at preindustrial levels. “Great,” you might think, “we’re home free!” However, climate is far more than just one globally averaged metric. Even though the average temperature stayed the same, there were still regional changes, with cooling in the tropics and warming at both poles (particularly in their respective winters):

There were regional changes in precipitation, too, but they didn’t all cancel out like with temperature. Global mean precipitation decreased, due to cloud feedbacks which are influenced by sunlight but not greenhouse gases. There were significant changes in the monsoons of south Asia, but the models disagreed as to exactly what those changes would be.

This intercomparison showed that even with geoengineering, we’re still going to get a different climate. We won’t have to worry about some of the big-ticket items like sea level rise, but droughts and forest dieback will remain a major threat. Countries will still struggle to feed their people, and species will still face extinction.

On the other side of the Atlantic, Damon Matthews and Ken Caldeira took a different approach. (By the way, what is it about Damon Matthews? All the awesome papers out of Canada seem to have his name on them.) Using the UVic ESCM, they performed a more realistic experiment in which emissions varied with time. They offset emissions from the A2 scenario with a gradually decreasing solar constant. They found that the climate responds quickly to geoengineering, and their temperature and precipitation results were very similar to the European paper.

They also examined some interesting feedbacks in the carbon cycle. Carbon sinks (ecosystems which absorb CO2, like oceans and forests) respond to climate change in two different ways. First, they respond directly to increases in atmospheric CO2 – i.e., the fertilization effect. These feedbacks (lumped together in a term we call beta) are negative, because they tend to increase carbon uptake. Second, they respond to the CO2-induced warming, with processes like forest dieback and increased respiration. These feedbacks (a term called gamma) are positive, because they decrease uptake. Currently we have both beta and gamma, and they’re partially cancelling each other out. However, with geoengineering, the heat-induced gamma goes away, and beta is entirely unmasked. As a result, carbon sinks became more effective in this experiment, and sucked extra CO2 out of the atmosphere.

The really interesting part of the Matthews and Caldeira paper was when they stopped the geoengineering. This scenario is rather plausible – wars, recessions, or public disapproval could force the world to abandon the project. So, in the experiment, they brought the solar constant back to current levels overnight.

The results were pretty ugly. Global climate rapidly shifted back to the conditions it would have experienced without geoengineering. In other words, all the warming that we cancelled out came back at once. Global average temperature changed at a rate of up to 4°C per decade, or 20 times faster than at present. Given that biological, physical, and social systems worldwide are struggling to keep up with today’s warming, this rate of change would be devastating. To make things worse, gamma came back in full force, and carbon sinks spit out the extra CO2 they had soaked up. Atmospheric concentrations went up further, leading to more warming.

Essentially, if governments want to do geoengineering properly, they have to make a pact to do so forever, no matter what the side effects are or what else happens in the world. Given how much legislation is overturned every time a country has a change in government, such a promise would be almost impossible to uphold. Matthews and Caldeira consider this reality, and come to a sobering conclusion:

In the case of inconsistent or erratic deployment (either because of shifting public opinions or unilateral action by individual nations), there would be the potential for large and rapid temperature oscillations between cold and warm climate states.

Yikes. If that doesn’t scare you, what does?

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Snowstorms and Sea Ice

Cross-posted from NextGen Journal

“That’s some global warming”, Fox News proudly announced. “Rare winter storm dumps several inches of snow across South.” It’s cold outside, and/or it’s snowing, so therefore global warming can’t be happening. Impeccable logic, or rampant misconception?

It happened last winter, and again so far this season: unusual snow and extreme cold thrashed the United States, Europe, and Russia. Climate change deniers, with a response as predictable as Newton’s Laws, trumpeted the conditions as undeniable proof that the world simply could not be warming. Even average people, understandably confused by conflicting media reports, started to wonder if global warming was really such a watertight theory.

But sit and think about it for a minute. If it’s cold right now in the place where you live, that doesn’t mean it’s cold everywhere else. It’s simply not possible to look at your little corner of the world and extrapolate those conditions to the entire planet. There’s a reason it’s called global warming, and not “everywhere-all-the-time warming”. Climate change increases the amount of thermal energy on our planet, but that doesn’t mean the extra energy will be distributed equally.

That said, an interesting weather condition has been prominent over the past month, telling a fascinating story that begins in the Arctic. At the recent American Geophysical Union conference in San Fransisco, the largest annual gathering of geoscientists in the world, NOAA scientist Jim Overland described the situation.

Usually in winter, the air masses above the Arctic have low pressure, and the entire area is surrounded by a circular vortex of wind currents, keeping the frigid polar air contained. Everything is what you’d expect: a cold Arctic and mild continents. These conditions are known as the positive phase of the North Atlantic Oscillation (NAO), an index of fluctuating wind and temperature patterns that impacts weather on both sides of the Atlantic.

The negative phase is different, and quite rare: high pressure over the Arctic forces the cold air to spill out over North America and Eurasia, allowing warm air to rush in to the polar region. Meteorologist Jeff Masters has a great analogy for a negative NAO: it’s “kind of like leaving the refrigerator door ajar–the refrigerator warms up, but all of the cold air spills out into the house.” The Arctic becomes unusually warm, and the temperate regions of the surrounding continents become unusually cold. Nobody visually depicts this pattern better than freelance journalist Peter Sinclair:

So what’s been causing this rare shift to the negative NAO the past two winters? In fact, global warming itself could easily be the culprit. Strong warming over the Arctic is melting the sea ice, not just in the summer, but year-round. Open water in the Arctic Ocean during the winter allows heat to flow from the ocean to the atmosphere, creating the high pressure needed for a negative NAO to materialize. Paradoxically, the cold, snowy weather many of us are experiencing could be the result of a warming planet.

An emerging debate among scientists questions which force will win out over winters in Europe and North America: the cooling influence of more negative NAO conditions, or the warming influence of climate change itself? A recent study in the Journal of Geophysical Research predicts a threefold increase in the likelihood of cold winters over “large areas including Europe” as global warming develops. On the other hand, scientists at GISS, the climate change team at NASA, counter that extreme lows in sea ice over the past decade have not always led to cold winters in Europe, as 7 out the past 10 winters there have been warmer than average.

Amid this new frontier in climate science, one thing is virtually certain: global warming has not stopped, despite what Fox News tells you. In fact, despite localized record cold, 2010 is expected to be either the warmest year on record or tied for first with 2005 (final analysis is not yet complete). What you see in your backyard isn’t always a representative sample.