CO2 is the biggest control climate knob. And it has been the biggest knob as far back as we can tell. So says Richard Alley. Actually it isn’t so much him saying this, as the latest science. Richard Alley is merely summarizing the science:
Steve Easterbrook has a great summary for those of you who don’t have 50 minutes to watch the above video (but really you should watch the video, it is mostly accessible to laypersons, and is very interesting).
But what do we say to people who say the lag proves current warming isn’t caused by CO2? We know that orbital changes (the Milankovitch cycles) kick off the ice ages – this was predicted 50 years before we had data (in the 1970s) to back it up. But temperature never goes far without the CO2, and vice versa, but sometimes one lags the other by about 2 centuries. And a big problem with the Milankovich cycles is that they only explain a small part of the temperature changes. The rest is when CO2 changes kick in. Alley offered the following analogy: credit card interest lags debt. By the denialist logic, because interest lags debt, then I never have to worry about interest and the credit card company can never get me. However, a simple numerical model demonstrates that interest is the bigger cause of debt (even though it lags!!). So, it’s basic physics. The orbits initially kick off the warming, but the release of CO2 then kicks in and drives it.
So, CO2 explains almost all the historical temperature change. What’s left? Solar irradiance changes, volcanic changes. When these things change, we do see the change in the temperature record. For solar changes, there clearly aren’t many, and they act lik a fine tuning knob, rather than a major control. 40,000 years ago the magnetic field almost stopped (it weakened to about 10% of its current level), letting in huge amounts of cosmic rays, but the climate ignored it. Hence, we know cosmic rays are at best a fine tuning knob. Volcanic activity is important, but essentially random (”if volcanoes could get organised, they could rule the world” – luckily they aren’t organised). Occasionally several volcanoes erupting together makes a bigger change, but again a rare event. Space dust hasn’t changed much over time and there isn’t much of it (Alley’s deadpan delivery of this line raised a chuckle from the audience).
So, what about climate sensitivity (i.e. the amount of temperature change for each doubling of CO2)? Sensitivity from models matches the record well (approx 3°C per doubling of CO2). Recently, Royer et al conducted an interesting experiment, calculating equilibrium climate sensitivity from models, and then comparing with the proxy records, to demonstrate that climate sensitivity has been consistent over the last 420 million years. Hence paleoclimate says that the more extreme claims about sensitivity (especially those claiming very low levels) must be wrong.
In contrast, if CO2 doesn’t warm, then we have to explain why the physicists are stupid, and then we still have no other explanation for the observations. If there is a problem, it is that occasionally the world seems a little more sensitive to CO2 than the models say. There are lots of possible fine-tuning knobs that might explain these – and lots of current research looking into it. Oh, and this is a global story not a regional one; there are lots of local effects on regional climate.
Note that most of these recent discoveries haven’t percolated to the IPCC yet – much of this emerged in the last few years since the last IPCC report was produced. The current science says that CO2 is the most important driver of climate throughout the earth’s history…
What I found fascinating about the talk was the way that Alley brought together multiple lines of evidence, and showed how our knowledge is built up from a variety of sources. Science really is fascinating when presented like this.
The three bold points are key.
First we have the fact that currently we can explain most of the warming going back a very long time, but not all of it. Sometimes the earth warms more than the models say it should. Does this mean that we are underestimating the importance of CO2? Maybe, but there are other ‘fine tuning knobs’ not represented in the models that could explain the mismatch. Research is being done in order to determine why this mismatch exists, but we would have had to have gotten many things wrong for there to be a major climate knob that we haven’t yet identified.
Secondly this line of evidence is not the main line of evidence used by the IPCC in attributing global warming to our GHG emissions. This is just another line of evidence that demonstrates that CO2 has a profound effect on the climate.
Thirdly, Richard Alley presents many lines of evidence to demonstrate his case. He builds a coherent picture, that fits well within the total body of knowledge on this subject and others that doesn’t rely on a single line of evidence. This is why something as trivial as the hockey stick debate, or climategate does nothing to undermine the science behind global warming.
UPDATE: Robert Grumbine has more details on why the CO2 lags, therefore anthropogenic global warming is a crock talking point is itself a crock.
[Deniers] want to conclude, or for you to conclude, that because in the ice age record temperature changes before CO2, that a) the current rise in CO2 is also caused by temperatures (sometimes citing the medieval warm period as the warm time that is causing the current rise in temperature) and b) that CO2 doesn’t have any affect on temperatures.
Have another look at the figure. See that point sitting way the heck away from all others? That’s the most recent value.
You don’t need a lot of scientific background to see that whatever it is that produced that CO2 value, it certainly was not the relationship that held for the other 798,000 years of the temperature – CO2 record. We can be more precise about it. The best fit line (the one that gives that nice high correlation) through the data points is CO2 = 266 + 8*T. So, for temperatures at the reference value, we expect a CO2 level of 266 parts per million. For temperatures 10 C below reference, we expect CO2 levels of 186 parts per million. Both of these accord fairly well with what we do see in the record — except for the most recent CO2 value. As you can see from the plot as well, there is indeed scatter around the best fit line. The standard deviation is about 11 ppm. That means we’re not surprised to see values 22 ppm away from the line (2 standard deviations), and, given 799 data points, we expect a few to be 33 ppm away. On the other hand, a value 9 standard deviations away — the case for the current CO2 levels — is ludicrous. Something must have changed.
We can turn this around, and ask: “If the relationship that held for the previous 798,000 years still did, what would the temperature need to have been to give the observed CO2 level — maybe CO2 is so sensitive to temperature that we simply have a one-time temperature observing problem?” Certainly the ice sheet temperatures do have their own observing issues. As I’ve mentioned, all data have problems. Still, same as we know that, we also know something about the size of the problem. So, turn the equation around and solve for the temperature that would correspond to the observed (modern) CO2 level. That’s a temperature anomaly of about 13 C (20 F) — equal to the entire range from the very warmest interglacial to the very coldest glacial! And that had to have occurred 600-1000 years ago, by the lead relationship. Meaning there had to have been an absolutely enormous warming (many times larger than even the highest medieval warm period values) and nobody noticed it … or else the previous relationship between temperature and CO2 broke down.
Since we know what did cause the CO2 rise — human activity — we’re not really surprised to see this answer. The old relationship did get broken. Rather than CO2 rising because the oceans released CO2 to the atmosphere, it is because humans have been burning fossil fuels and making cement. But we do like to be able to arrive at our conclusions from different directions. Here, we need only to look at the temperature and CO2 values themselves to see that the relationship that used to hold has broken down in the modern day. Don’t need to know the first thing about isotope geochemistry (which is explained nicely in that faq by Jan Schloerer) to see this.
Yet despite having been shown to be wrong, this myth persists. And that is why deniers are anything but skeptics.