Global Warming Does Not Cause Climate Change; The Fever Does Not Cause The Disease; A Low Fever is Not A Cure
Did you panic in 1998? Because, remember, that at the end of the year the global mean surface temperature (“GMST”) record looked like this:
Look at the 15 years from 1983 – 1998, and based on this chart, estimate what 2013 would look like. I would say that it would not be unreasonable to be concerned that some tipping point had been reached and the climate would lurch into ultra-hot territory.
Here’s what actually happened:
If you construe the world as a battle between two sides, one which wants to restrain carbon emissions and one which doesn’t, this is good news for the laissez faire folk. “Look!” they squeal. “It stopped!” they scream. “There is nothing to worry about!” they holler. “We win” they trumpet.
But this is crazy thinking. Most of us should be thinking about what the future holds and how we should best deal with it. There are no “sides”, one hopes. We all want a safe, civilized, decent world. If there are threats we need to cope with them, and if a threat turns out not to materialize, so much the better.
The question is how we should look at this hiatus.
Part of the answer is that it really is good news. The gun whose barrel we thought we were looking down in 1998 turned out not to be there. It’s silly not to be somewhat relieved.
Part of the answer is that we are overvaluing the hiatus in exactly the same way we overvalued the spike. In 1998, at the height of the El Nino, it was easy to imagine a world in imminent danger. In 2013, after a few quiet years, it is easy to imagine that we’re out of the woods, that the problem is totally overstated. Greenhouse gases have been accumulating over the last 15 years, and a 15 year window on global temperature shows nothing much happening. But in general the eye picks out the last part of a graph much more than most statistical approaches do.
What I’d like to emphasize, though, is that both in the scary picture of 1998 and the relatively less scary picture in 2013, people are overvaluing the importance of GMST for a peculiar cultural reason. That reason is that we decided to call the carbon dioxide issue “global warming”, and it’s a misnomer.
Why Greenhouse Gases Matter and CO2 Matters Most of All
The present discussion needs only the most rudimentary understanding of global warming:
That’s close enough for now. The greenhouse gases are building up, making it harder for the infrared heat radiation of the earth to escape into space. This causes the world to heat up to approach the equilibrium that the new greenhouse gas concentrations require.
Most greenhouse gases (including water vapor) are short-lived, and so the amount in the air is a rough equilibrium between the emissions and absorptions. Carbon dioxide (CO2), on the other hand, while freely exchanged between the ocean and the atmosphere, requires a very long time to exit the system. Conceptually, it is not a terrible approximation, on the timescale of a human lifespan, to say that it doesn’t go away at all; at least not on its own.
Why it matters is quantitative.
Our best understanding of the amount of fossil fuel there is available and how CO2 works leads us to an expectation that burning all that fuel will cause a severe disruption of all natural systems (even disregarding the many other insults we are visiting on Nature) and many human systems as well. This understanding is not primarily based on observations, nor on complicated computer models, but on our theoretical understanding of physics and on paleontological evidence. This understanding traditionally is boiled down to a number, “the sensitivity”. Now, in general, “a” sensitivity is the ratio of an output to an input in a system. An electrical engineer would refer to the “gain” or “amplification factor”. In our case “sensitivity” is a measure of this question: if we put in so much change in CO2, how much global temperature change will we get out?
There are numerous ways of getting at this, but in general the estimates pile up between 2 and 3, that is, between two and three degrees of warming (Celsius) of the atmosphere-ocean-sea ice system per doubling of CO2 (with water vapor as a variable quantity responding to the changed forcing). If this number is right, and we burn all the fossil fuels, the earth’s temperature record over the long run of human history will end up looking like this (this is Joe Romm’s extension of Marcott et al’s data (note, plot is in degrees Fahrenheit):
Everything on the two graphs above is compressed into that little uptick at the end of the blue part of the line.
And as we pointed out before, if the estimated sensitivity is as much as two times overestimated, the less alarming picture looks like this:
which just isn’t all that reassuring.
Now a bunch of people are asking you to bet the farm that the sensitivity is actually zero (that blue observational spike actually could conceivably be an artifact). Indeed if the only evidence you had was a GMST record truncated before 1998, your best bet would be very close to zero.
Could the Sensitivity be Zero?
There is one thing we can be sure of. We cannot double the CO2 in the atmosphere without changing the climate. This is a very large perturbation compared to the ones ordinarily seen in nature. The system must respond because if it didn’t there would be a violation of the Law of Energy Conservation to the tune of 4 watts per square meter, or about (if I got my sums right) two billion megawatts. The reasonable expectation is that the surface heats up somehow. Perhaps mostly on land or firstly on land; that also is part of the expectation.
But other things could conceivably happen. Perhaps somehow the extra opacity of the atmosphere causes thick low clouds to form, having little greenhouse effect and much reflective capacity. Maybe a natural thermostat kicks in. There is little evidence for such a thing, but it’s not inconceivable.
In a more extreme scenario there is a sudden melt of Greenland, so cooling the North Atlantic as to cause a mini-ice age lasting a few hundred years. This scenario is no longer given much credence, but it was very much on the table for a while, and there is a paleoclimate precedent for exactly that happening, the Younger Dryas cooling. People living through such a period would consider the sensitivity of the climate system to be negative, for practical purposes.
But notice that these scenarios are still linked with very large climate changes.
Could the Sensitivity be Near Zero and Nothing Important Change about Climate and Everything Turn Out Great?
Yes it could.
And you could spend your last ten bucks on lottery tickets and you could win. It just isn’t a good plan.
Could We be Barking Up the Wrong Tree?
Before climate change became obvious, when it was merely a prediction, the sensitivity was a good thing to focus on. (There still are good reasons to think about it.) The presumption was that, all else equal, the hotter scenarios were responding more sensitively than the less hot ones. The scale separation between climate models and actual impacts is too big for most applications. It was hard to know what so much warming implied for climate change in real scenarios.
But this got us into backwards thinking and a backwards way of speaking. We started to speak as if global warming causes climate change, as if the number of degrees of warming were diagnostic in some sense of what would happen to us. Being humans we got wrapped up in the symbol and forgot the reality.
Greenhouse gases cause radiative transfer processes to change. These cause energy to accumulate in the system as it seeks a new equilibrium. The climate changes, in turn, to respond to the redistributed energy. And one of the many many consequences is, probably, an increase in GMST. But there may be other consequences we care about!
The real sensitivity we care about is damage per unit of carbon emitted. And the damage is caused directly by climate change, not by GMST.
Consider most of the bizarre events of the past few years – the Australian megadrought, the Russian fires, the Pakistan floods, the Texas heatwave, Sandy, the super-summer and the endless winter recently in the midwest. Each of these was associated with a phenomenon called “blocking” wherein the jet stream develops huge, sluggish meanders, delivering “the wrong air at the wrong time” to some large area. There is considerable evidence that this phenomenon has become more prevalent in recent years. It is especially associated with an increase in local extreme heat events.
Notice that one way the system can avoid increasing its average temperature is by making the temperature more unevenly distributed – an extremely hot place far to the pole can radiate so effectively as to more than balance out a comparably cool place near the pole. This is in fact one sort of climate change that fills the bill of equilibrating the energetics without changing the mean temperature.
It’s pretty clear that even if global warming has in some sense “gone away” or “on hiatus”, the world is no longer producing the reliable weather that it has over the period of human history. Most likely, this is only the beginning. So it’s a bit crazy to still be quibbling about sensitivity in policy circles. It really is starting to hit the fan.
Global Heating is Still Happening
In fact, it would be better if we still had a country that was willing to actually put instruments in space to watch the earth. A satellite (TRIANA, later called DSCOVR) to observe the global energy balance was proposed and built and mothballed an unbelievably long time ago, in 2001. Its chief proponent had political enemies. His name was Senator Al Gore.
It’s currently scheduled for launch in January of 2014. A pity we are going to miss 13 years of data.
But the biggest clue is a very simple one: sea level.
The sea rises for two reasons: 1) there is more water in it 2) it gets warmer. Since the way the ocean gets more water is from melting ice, something is getting warmer.
And this is the problem for those who want the “problem went away” argument to be taken seriously. If there is no greenhouse effect, if what we are seeing is unforced natural variability in the atmosphere, then the extra heat in the atmosphere must be coming at the expense of the ocean. (In the El Nino cycle, the ocean cools in the years the atmosphere warms, and vice versa). There is no sign at all of cooling in the ocean, though, and indeed the evidence is that the ocean is warming as well. This pretty much takes natural variability off the table. (I heard this argument from Isaac Held).
It’s tempting, then, to say “global warming has not stopped, it has just gone underwater”, but I think this is an opportunity to let go of the always poorly chosen name “global warming”.
I believe John Holdren coined the term “climate disruption”, and I think it is exactly right. Our activities are rocking the boat in no small way. The climate system is more than just one number, however interesting. For now we need to look up and notice that even though the global temperature has been relatively steady for a while, there is no sign whatsoever that rapid climate change is abating.
Update: I would be remiss if I failed to point out that Chris Colose has disagreed with my claim here. I haven’t met him, but based on what I’ve seen of Colose and his writings online, ordinarily I would defer to him on climate dynamics. However, I think there is at least one difference in shade of meaning here (I suspect there are a couple) and I stand by my point.