Very generally, the direct causation between carbon and climate goes like this:
First, global warming causes global warming. It gets hotter, especially at or near the poles. Temperate areas see more punishing heat waves in the summer. A few areas in Canada and Siberia might benefit from this; most regions will not. The poorer tropical countries in particular really don’t need to be any hotter than they already are.
Second, changes in the greenhouse effect will alter the temperature differences in the air and ocean that cause the system of currents that govern our weather. These currents are produced by convection, which is when warm gases or fluids rise, pulling in others to take their place. Some currents, like the global ocean flows (thermohaline currents) that make us forget that Hamburg is at the same latitude as Edmonton, are fairly stable. Others, like those that cause the oscillation between El Niño and La Niña over the Pacific, are highly sensitive. But all of them could be altered if global temperatures change enough.
Third, changes in air and ocean currents cause precipitation patterns to change. Some regions, like the US southwest, will experience chronic drought, while others will get heavier rains, such as more intense monsoons.
Finally, the combination of changed convection patterns and warmer oceans will likely cause more powerful storms. Once in a century storms will hit once in a decade, and once in a decade events will become commonplace. This is really beginning to concern insurance companies.
Meanwhile, when the climate changes, so do terrestrial ecosystems. Many forests, for instance, will become unsustainable and evolve into grasslands or savanna. “Evolve” is possibly not the right word: the transition will often take the form of raging forest fires. Species, lovable like polar bears (not so lovable at close range) and less charismatic like thousands of varieties of plants and insects, will become extinct. It’s difficult to predict what the long-range effect of ecosystem changes will be on us humans.
A different impact that has gotten a lot of attention is sea level rise. This will happen for two reasons. First, when you heat water it expands. This effect is absolutely certain, and it provides the basis for conservative predictions of sea-level rise—less than a meter over the course of the twenty-first century. More speculative is the effect of melting glaciers and ice sheets that sit on top of land masses. (Only ice with land under it raises sea levels when it melts.) To be precise, the progressive melting of major ice formations is fairly certain as the earth warms, as underscored by the recent findings concerning the massive West Antarctic ice sheet, but it will probably take a century or more before these larger impacts are felt. The West Antarctic melt appears to be irreversible, and it guarantees an additional sea level rise of 12-15 feet—several hundred years from now.
Equally worrisome, however, is a quite different ocean event, a progressive, unstoppable decline in its pH. Chemical reaction between the increasingly carbonized atmosphere and ocean surfaces is causing ocean water to become more acidic. This is already a problem for commercial shellfish growers in my region, and it is likely to lead to the disappearance of the world’s coral reefs. Worst case scenario: as acidification progresses plankton, the tiny (really tiny) shellfish on which the marine food chain depends, could collapse.
Now on to the serious stuff: feedback loops. Human-induced (anthropogenic) climate change will produce various side effects that can either dampen or amplify the original effect of feeding additional carbon into the global carbon cycle. One possible negative feedback would be increased cloud cover. When you look down on them from an airplane, clouds are white, which means they reflect most of the light streaming at them from the sun. The technical term for this is that they strengthen the earth’s albedo. At one time scientists thought this might be a useful counterforce to human carbon-spewing. Currently, however, the view seems to be that clouding over will not be our salvation. So it goes.
So let’s worry about the positive feedback loops. One that is probably already kicking in is the melting of the glaciers and ice caps themselves. After all, they’re white, and after they disappear, whatever is underneath them is darker. The biggest effect so far is likely to be the dramatic summer melting of arctic ice. It’s great for shipping but not good in the way it reinforces climate change.
Albedo feedbacks are small change, however, compared to the really, really massive potential embodied in stored methane. Recall from two posts ago that, while some organic carbon was sequestered over the millennia in the form of fossil fuels, another portion was stashed away as buried or frozen methane. (To be precise, methane is natural gas, but the methane deposits we’re talking about now are not recoverable with current technologies. Fortunately.) How much methane are we talking about, and how does it compare to fossil fuels? Let’s take a look:
The key point is the relationship between stored methane and carbon stored in fossil fuels. As you should realize by now, the majority of fossil fuel deposits need to remain undeveloped if we want to avoid catastrophic climate impacts. The precise amount is a point of dispute, but the IPCC, governed by a consensus process, says we should burn no more than 600 billion additional tons, and that’s less than 30% of the sum of all known fossil fuel reserves—leaving out resources altogether.
Now look at the methane. Adding up ocean clathrates and peat deposits, it comes to about 27% of all potentially recoverable fossil fuels and about 1.8 times known reserves. Here are two things you want to know right away. One is that the fossil fuel budgets the IPCC and other scientific bodies propose are based on the assumption that none of this methane will escape and enter the carbon cycle. The second is that, whatever the source, when a fossil fuel is burned it releases CO2, but a ton of methane release has about twenty times the greenhouse effect as a ton of CO2 because it targets a range on the light spectrum that CO2 misses. In other words, if just a relatively small proportion of stored methane is released into the atmosphere, no matter how diligent we are in our own carbon budgeting, all bets are off.
As you can see, about 400 billion tons of carbon are biding their time in peat bogs. This alone is equal to almost a fifth of all fossil fuel reserves, it is subject to the 20x amplification factor, and it doesn’t take all that much heat to release methane from a peat bog. In fact, it’s probably begun to happen on a small scale and will increase (uncontrollably) as the earth continues to warm. One of the reasons the IPCC and others want to set a 2ºC limit on average temperature rise is to prevent most of this peat methane from escaping.
But now take a look at its elder brother. A carbon vault more than half again larger than all the world’s reserves of fossil fuels can be found in frozen methane deposits, known as clathrates, located in portions of the ocean that are near enough to the land to capture lots of nutrients but deep enough so that the methane will condense and not rise to the surface. I should add, some of these methane deposits are also found in deeper freshwater bodies at arctic and subarctic latitudes. Well, what do you suppose might happen as the greenhouse effect gets into high gear? The risk is that the most vulnerable clathrates, the ones closest to the temperature tipping point at which they will expand and rise to the surface, will do exactly that. Being methane—fierce greenhouse instigators—they will quickly kick global warming into an even more advanced phase. And this will tip the next layer of clathrates, and so on. The result would be an immense increase in greenhouse gas concentrations over a relatively short period time—perhaps measured in years rather than decades, much less centuries. It would be alligators in the arctic all over again. This, friends, is the risk of runaway climate change.
How likely is this to happen? What’s the critical temperature increase that could set off such a process. Damned if I know. In fact, no one really knows. There are teams of scientists monitoring arctic clathrate sites. They have reported burps but not yet belches, if that’s reassuring. Authorities in this field say that the likelihood of a catastrophic clathrate event remains small, but it’s not zero, and whatever the level of risk, it will increase as global temperatures increase. This is the scenario that scares the bejesus out of everyone who studies it, although there’s no point in being paralyzed by the fear of something that hasn’t happened yet and might well never happen at all. On the other hand, a small, measured dose of energizing fear could be quite valuable.
Incidentally, there is some evidence that massive, sudden clathrate releases have occurred at other points in earth history.
The final thing to bear in mind—and this is either good news or bad news depending on how you look at it—is that there is a very long lag between human activities that add carbon to the carbon cycle and impacts felt by those humans. It takes decades for carbon to make its way into the atmosphere where it can do its heat-trapping thing. Many of the processes this sets in motion, like the melting of ice sheets, take decades longer. It is said that climate change is not in the future, but now: we are seeing hotter weather, longer droughts, bigger storms. This is true, but these are the result of fossil fuel extraction that occurred decades ago, back in the days of Vince Lombardi and Mick Jagger. (Hmmm.) What we’re doing today will alter the world of our children and grandchildren.
If you’re an economist and you live and breathe present value calculation, that’s a benefit, since any bad stuff that happens a century from now is much less costly after you discount it back. If you worry about the political capacity of contemporary society to act on the basis of consequences that won’t be felt until today’s citizens are dead and gone, it’s not so wonderful.
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4 comments:
Minor editorial point: when you discussed albedo you incorrectly identified it as a "negative feedback."
A near term problem in terms of the global politics of this is that the two largest CO2 emitters by far, China and the US, are both not only using very low discount rates to consider future effects of their behavior, but in the very near term and up to about a degree or so increase in global average temperature (at least F not C), their GDPs gain on net, even though they are losing in certain sectors.
How is this? It is the reduction of winter heating bills, and there is little doubt that while certain winter sports are hurt by warming, lots of people go south in the winter (in the US) and warmer temperature continues to have a positive coefficient in most models of permanent migration in the US. The list of costs of higher temps right now is long, but for at least another degree increase or so, that lowering of winter heating bills outweighs it, which somewhat weakens the push for doing anything about this in the US (and China, where the same pretty much holds) in the near term, quite aside from the fears of job loss by those in the fossil fuel industries.
Note, I am not approving of any of this, simply noting the hard reality of it.
Thanks for the correction, Barkley -- much appreciated. You'll notice that I'm not taking up the various "costs" as integrated assessment models identify them. This is partly because, in light of the bigger picture, I don't think they are first-order, big as they may prove to be. Second, possibilities for and limits to adaptation really throw a question mark over most of them. Agriculture is a key example. Farmers can grow new crops with new seed varieties and use new practices. What effect will this have on near-term and long term agricultural impacts? Difficult to say.
I fixed the glitch Barkley noticed: positive feedback is now positive.
And of course I had a glitch in my comments, identifying PRC and US as using "very low discount rates." That should have read NOT using very low discount rates.
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