In the last few days I’ve made the mistake of engaging, online, with climate change deniers. It’s incredibly frustrating and I know that I really shouldn’t. Sometimes, though, I just can’t help myself. The problem is that those who deny that climate change is influenced by man are adamant that they’re correct. They make strong, absolute statements: “complete nonsense”, “absolute rubbish”, “the evidence has proven the climate models are wrong”. This is no way to engage in a discussion and makes it impossible to engage in any sensible way.
What makes it difficult is that many seem to have a basic knowledge and understanding of science and use this to back up their statements. They know that CO2 is a weak greenhouse gas and claim this means it can’t be influencing the climate in the way the models indicate. They ignore that, at the moment, CO2 is increasing so fast that it is likely the dominant gas as far as global warming is concerned. They take the surface temperature data for the last 15 years and claim that since there hasn’t been a statistically significant increase, that it proves warming has ended and that the models are wrong. They ignore that 15 years is too short a timescale to make statistically significant measurements and that it could well have risen, we just can’t say that definitively.
The real problem seems to be how they twist the scientific method. The scientific method, in a simple sense, requires that models and/or theory need to be confirmed by observations and/or experiments. If the models don’t match the observations it does mean that there are problems with the models. It doesn’t mean that there is only one alternative. It’s not simply between man-made climate change and non-man-made climate change. It’s clear that humans have released a large amount of CO2 into the atmosphere in the last century. This is a greenhouse gas and must be having an effect on our climate. The question is how big is this effect, not whether it exists or not. The other, related, question is how big can this effect get if we continue as we are.
Another issue is that the typical criticism is that the models predict larger surface temperatures than are currently observed. I suspect that this mis-represents what the models actually do. I suspect that the models actually work with energy, not temperature. We receive a very well known amount of energy from the Sun. Some is reflected (albedo) and some is absorbed. The Earth then re-radiates this energy (at a longer wavelength). This is then transferred through the atmosphere and eventually out into space. The Earth is in equilibrium if the amount of energy received matches the amount that is reflected and radiated back into space. If the Earth receives more than it radiates/reflects it heats up. If it reflects/radiates more than it receives it will cool down.
One way these models could work is to consider the composition of the atmosphere, the Earth’s albedo and then use the incoming radiation to calculate what surface temperature is required for the Earth to be in equilibrium. This, however, is time-independent and doesn’t tell you how long it will take to reach this equilibrium temperature. It also doesn’t tell you how this change in temperature influences atmospheric composition (i.e., it ignores feedback). The alternative (and what I suspect is done) is to consider time-dependent calculations. This, however, is extremely complicated. If we’re not in equilibrium then the excess energy can be used to heat the oceans, heat the surface and atmosphere, and/or melt the polar ice caps. This can take time as the amount of energy required to change the temperature of the oceans or melt the polar ice caps is substantial and differs significantly from that required to heat the atmosphere or the surface. Getting this slightly wrong could significantly change the time evolution of the surface temperature. This could also change weather patterns, like the jet stream or the gulf stream. These changes could also influence how the polar caps melt or how the energy is transferred through the oceans. Running such time-dependent models is extremely difficult and that they don’t exactly match observations is not that surprising.
In a sense what is more important is the energy budget. Are the changes to the composition of our atmosphere requiring that the surface temperature of the Earth rise in order to re-establish equilibrium. There are, in fact, measurements suggesting that currently we aren’t actually in equilibrium and that we receive more energy than we lose. This is shown in the figure below (Murphy et al., 2009, JGR, 114, D17107) which suggests that this has been the case since 1970. If this is correct, then the surface temperature has to rise. There is really no alternative. We just don’t necessarily know how long this will take.
This energy imbalance is also consistent with the current situation in which the CO2 levels have been rising without a related rise in surface temperature. If there was no energy imbalance and the CO2 levels had risen without a corresponding surface temperature rise, it would suggest that CO2 was not an important greenhouse gas. That there is an imbalance tells us that the increased CO2 is indeed trapping more energy. This energy must be going somewhere and it appears to be going into heating the oceans and melting the ice caps.
If this energy imbalance is correct, at some point the surface temperature will have to rise. It may be difficult to determine exactly when. It may also be difficult to determine exactly the consequences. Will the melting in the polar regions release methane which is a much more effective greenhouse gas than CO2. Claiming, however, that because the models don’t exactly match observations does not then imply that global warming is not being influenced by man.
This post has got slightly longer than I intended. I was really just wanting to express how frustrating it is to engage with those who claim that our releasing large amounts of CO2 into the atmosphere is not having an impact on the climate. Maybe the models will end up being wrong, but it’s not going to be because climate scientists are making the silly mistakes that climate change deniers claim that they are.