There are a few things I have been considering writing about, but since I’ve renamed the blog to be more explicitly physics orientated, I thought the first should at least reflect that change. There’s a recent paper by James Hansen – and 17 co-authors – called Assessing ‘‘Dangerous Climate Change’’: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature. It’s an interesting paper because it discusses both the scientific evidence for global warming/climate change and considers various policy options (violating what many would regard as a fundamental rule – I don’t though). The paper’s been discussed in a number of other places already (here, and here for example), but I was going to discuss one very basic thing that I think it covers very well.
Probably the most fundamental aspect of global warming is the existence of an energy imbalance. If the earth is in energy deficit (i.e., it is gaining more energy than it loses) global warming, by definition, is happening. As the paper says
At a time of climate stability, Earth radiates as much energy to space as it absorbs from sunlight. Today Earth is out of balance because increasing atmospheric gases such as CO2 reduce Earth’s heat radiation to space, thus causing an energy imbalance, as there is less energy going out than coming in. This imbalance causes Earth to warm and move back toward energy balance. The warming and restoration of energy balance take time, however, because of Earth’s thermal inertia, which is due mainly to the global ocean.
The paper then discusses how one might determine the energy imbalance. It’s a reasonably tricky thing to determine, but the most promising is to calculate the rate of changing heat content of the ocean, atmosphere, land, and ice, with the oceans being critical as they store 90% of the excess energy. The paper concludes that
Argo data reveal that in 2005–2010 the ocean’s upper 2000 m gained heat at a rate equal to 0.41 W/m2 averaged over Earth’s surface. Smaller contributions to planetary energy imbalance are from heat gain by the deeper ocean (+0.10 W/m2), energy used in net melting of ice (+0.05 W/m2), and energy taken up by warming continents (+0.02 W/m2). Data sources for these estimates and uncertainties are provided elsewhere. The resulting net planetary energy imbalance for the six years 2005– 2010 is +0.58 +- 0.15 W/m2.
So there we have it. Estimates based largely on observations and measurements suggest that the Earth’s climate system accrued energy at a rate of 0.58 +- 0.15 W/m2 over the period 2005 – 2010. Fairly conclusive I would suggest. This of course doesn’t necessarily tell us what’s causing this energy imbalance. The paper, however, does say
If the sun were the dominant forcing, the planet would have a negative energy balance in 2005– 2010, when solar irradiance was at its lowest level in the period of accurate data, i.e., since the 1970s. …. The full amplitude of solar cycle forcing is about 0.25 W/m2, but the reduction of solar forcing due to the present weak solar cycle is about half that magnitude as we illustrate below, so the energy imbalance measured during solar minimum (0.58 W/m2) suggests an average imbalance over the solar cycle of about 0.7 W/m2.
So, more evidence that it’s really not the Sun and, if anything, the influence of the Sun means that the energy imbalance for the period 2005-2010 is slightly below the average for the solar cycle.
The paper also mentions anthropogenic aerosols and says
Earth’s measured energy imbalance has been used to infer the climate forcing by aerosols, with two independent analyses yielding a forcing in the past decade of about -1.5 W/m2, including the direct aerosol forcing and indirect effects via induced cloud changes. …… Increase of Earth’s energy imbalance from reduction of particulate air pollution, which is needed for the sake of human health, can be minimized via an emphasis on reducing absorbing black soot, but the potential to constrain the net increase of climate forcing by focusing on black soot is limited.
So, the conclusion is that anthropogenic aerosols are actually acting to reduce the energy imbalance. As we reduce anthropogenic aerosol emissions, the energy imbalance will increase, increasing the rate of global warming. Maybe we can reduce this slightly by also reducing the amount of black soot (that absorbs energy), but this is likely to not completely counteract the influence of the reduction in anthropogenic aerosols.
So, why have I written this? Well, because the existence of an energy imbalance is fundamental to anthropogenic global warming (AGW). If you think AGW is not real, then there are a number of things you need to be able to show. Firstly, that here’s something wrong with the observations that are used to determine the energy imbalance. Anything’s possible, but just stating that it’s difficult isn’t really sufficient to conclude that the observations are wrong. Secondly, you need to show what’s wrong with fundamental radiative physics. A fairly simple calculation could show that the energy imbalance that we have today is consistent with what we’d expect from basic radiative physics (maybe not trivial, but not that hard). So, if AGW isn’t real you need to both show that there’s something wrong with the observations, and that there’s something fundamentally wrong with basic physics. Anything’s possible I guess but that doesn’t make it likely.
One could argue – I guess – that maybe the observations are right (i.e., there is an energy imbalance), but that it’s natural, not anthropogenic. I’ve already discussed (above) why it can’t be the Sun. It also can’t be some kind of natural cycle (such as Wyatt & Curry’s Stadium Wave). Why? Well, we have plenty of evidence (paleo-climatology) to suggest that the global surface temperature is higher, today, than it’s been for centuries, or even longer. If it’s some kind of natural cycle, then the energy imbalance should vary from being negative (i.e., the Earth should be losing energy when surface temperatures are near the maximum) to positive (i.e., the Earth should be gaining energy when surface temperatures are near the minimum). It really can’t be some kind of natural cycle if the energy imbalance is still positive when the surface temperatures are as high as they are today. That, of course, also ignores that we have no physical mechanism for producing a natural cycle that can produce what we’ve observed with respect to global warming.
Anyway, this has become a rather long post to try and explain why I think understanding basic physics is a crucial part of understanding anthropogenic global warming. If you dispute AGW, either you’re about to become one of the world’s most famous scientists (by, likely, rewriting basic physics) or you don’t understand physics sufficiently well to really have any strong opinions with respect to AGW. As usual, if others disagree or think I’ve got something wrong (as I may well have) feel free to point that out through the comments.