Some more Climate Change

Maybe I should give up writing about this, but I wanted to expand on something I tried to write about before. There’s been a claim that there has been no warming since 1997. Firstly, this statement is wrong. The correct statement is, if one considers the temperature anomaly data since 1997, one cannot make a statistically significant claim about the warming. One cannot say, with any certainty, that there has been warming. One can also not say, with any certainty, that there has not. I do argue, in an earlier post, that one can make a statistically significant statement about warming over the last 17 years, but I won’t go into that here. In some sense, however, focusing only on the surface temperature is missing the point. Global warming is really about whether or not we are receiving more energy from the Sun than we radiate back into space. It’s not simply about whether or not the surface temperature has risen.

So can we actually measure if there is an energy difference or not. The answer is yes. A satellite called the Earth Radiation Budget Satellite (ERBS) operated from 1984 to 2005. Similar instruments were carried on some of the NOAA satellites. Since 1997 an instrument called Clouds and Earth’s Radiant Energy System (CERES) has been carried on various NASA satellites. This means that we can measure, directly, the difference between the amount of energy the Earth receives (from the Sun) and the amount radiated back into space. Shown below is a figure from Loeb et al. (Nature Geosciences, 5, 110-113, 2012). The bottom panel shows the energy budget (in W m-2) while the top panel compares this with the accumulation of energy in the Earth’s oceans. From the bottom panel it is clear that, since 2001, the Earth has been receiving an excess of energy of about 0.5 J m-2 every second. This corresponds to an increase in energy of 1022 Joules every year.

Energy budget figure from Loeb et al. (2012)

Energy budget figure from Loeb et al. (2012)

Shown below is a figure from a paper by Richard Allan from Reading University (Allan R., Meteorological Applications, 18, 324-333, 2011) and illustrates the energy budget going back to 1985. This suggests that we’ve been gaining energy since just after 1990. A paper by Murphy et al. (JGR, 114, D17107, 2009) suggests that this may have started as early as 1975. Furthermore, the figure below also shows comparison data from the Coupled Model Intercomparison Project Phase 5 (CIMP5). These model results appears to match quite well with the observations and suggests, to me at least, that the climate models do a pretty good job of predicting the Energy budget. What I was really wanting to highlight is that real data suggests that we have been receiving excess energy from the Sun for at least 20 years, maybe longer.

Energy budget graph from Allan (2011)

Energy budget graph from Allan (2011)

So why is this important? Well, the existence of this energy difference proves that global warming is taking place. Global warming simply means that we are getting more energy from the Sun than we lose. This has to lead (eventually) to a rise in the surface temperature in order to regain equilibrium, but even if this rise has not yet happened does not mean global warming is not happening (and I’m not suggesting that the surface temperature is not rising).

Can we go further and interpret this in a bit more detail. The satellite data shown above suggests that the Earth is receiving more energy from the Sun than it is losing back into space. It also appears that the excess has existed for at least 20 years, maybe longer. Could it simply be that the amount of Solar energy that is being received by the Earth has increased. No, the Sun’s luminosity does change but it varies with an 11 year cycle and the last minimum was in about 2008 (see for example Lockwood, Proc. Roy. Soc., 2009). So, it can’t simply be that we’re just intercepting more energy from the Sun. Could it be that the Earth’s surface is simply radiating less energy. No, the surface temperature today is higher than it was 20 years ago (and the surface area of the Earth has clearly not changed) so the Earth’s surface, if anything, is radiating slightly more energy today than it was 20 years ago. The logical reason for why we have an excess of energy is that the Earth’s atmosphere is preventing some of the radiation, that would have been able to escape 20 or so years ago, from escaping.

How can this be? Well the one obvious answer is that the increased level of CO2 (which is a greenhouse gas) is trapping some of the radiation and reducing the amount of energy that can leave the Earth’s atmosphere. So, what’s happening to the excess energy? Why doesn’t it simply increase the surface temperature of the Earth? It will eventually but it appears (according to the top figure above) to be going into the oceans. The oceans can absorb a lot of energy without the temperature changing significantly. It may therefore be a while before we see the rise in surface temperature that will be needed to close the energy budget.

Anyway, I was trying to keep this reasonably short, so let me summarise what I was trying to get at in this post.

  • Real data suggests that we have been receiving more energy from the Sun than we lose back into space and that this excess has existed for at least 20 years.

  • This excess energy cannot be because we are intercepting more energy from the Sun. The Sun’s luminosity was decreasing from 2000 till about 2008.

  • This excess energy cannot be because the Earth’s surface is radiating less energy. The surface temperature is higher today than it was 20 years ago.

  • The likely cause is that the increase in atmospheric greenhouse gases (CO2) is trapping radiation that would, in the past, have been able to escape.

In some sense, what I am trying to do here is to focus on the basic science. If we are receiving more energy from the Sun than we lose, then global warming is happening. This is irrefutable. There should be no debate about this. One could discuss the validity of the data, but not the validity of the interpretation. This has persisted for more than one Solar cycle, so it can’t simply be that we’re intercepting more Solar radiation, and the surface temperature of the Earth has not decreased, so this difference is most likely because of an atmospheric greenhouse effect. The atmosphere must be trapping some energy that would have escaped in the past. The most likely cause is the increase in CO2 driven by our use of fossil fuels. This could be open to some interpretation (maybe changes in cloud cover could influence our albedo) but given that the climate models shown in the second figure above do a good job of matching the data, it will take quite some doing to find a viable alternative.

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One thought on “Some more Climate Change

  1. Pingback: Global warming – the basics | To the left of centre

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