|This graph shows the distribution of extra energy that the climate system has accumulated due to the radiative imbalance created by the dramatic increase of greenhouse gases in the atmosphere since the Industrial Revolution. Note which component has stored the most energy: the oceans - far more than the continents or atmosphere. The reason for this is simple: the heat capacity of water (see here and here) vs. the heat capacity of land or the atmosphere. Water can absorb 1000 times the energy that the atmosphere can. While land and near-surface temperatures have been increasing, that increase has been greatly moderated by the ability of water to absorb the energy as the atmosphere interacts with it. Put another way, if water wasn't so effective at absorbing energy, more heat would be present in the atmosphere today than it is. This heat absorption has implications for our future, which I will come back to later.
This information can be viewed in another way:
Graph courtesy of Skeptical Science: "Total Earth Heat Content [anomaly]" from 1950 (Murphy et al. 2009). Ocean data taken from Domingues et al. 2008.
Again, total ocean heating in the past 50 years exceeds by a large margin the heating that has taken place on land and in the atmosphere. Globally averaged surface temperatures are 0.74C (+-0.18C) warmer as of 2005 than they were in 1906 (IPCC). What the excess heat content in the ocean means is over the course of thousands of years into the future, global temperatures will continue to rise until the excess energy is re-radiated back to space. The climate system will find a new stable state eventually, but as far as our species and today's ecosystems are concerned, temperatures will continue to rise: the physics demand nothing else.
Next, let's look at how the temperature observations can be explained:
Source: IPCC WGI Technical Summary Figure TS.23: (a) Global mean surface temperature anomalies relative to the period 1901 to 1950, as observed (black line) and as obtained from simulations with both anthropogenic and natural forcings. The thick red curve shows the multi-model ensemble mean and the thin lighter red curves show the individual simulations. Vertical grey lines indicate the timing of major volcanic events. (b) As in (a), except that the simulated global mean temperature anomalies are for natural forcings only. The thick blue curve shows the multi-model ensemble mean and the thin lighter blue curves show individual simulations. Each simulation was sampled so that coverage corresponds to that of the observations.
This graph (b - bottom portion) shows that natural forcings alone cannot explain the observed temperature trend since 1900. Significant divergence starts to occur starting in the 1960s and gets more extreme with time. If anthropogenic forcings (i.e. human causes) are included in simulations, the temperature record can be explained. This provides solid evidence that our greenhouse gas pollution is having a demonstrable effect on the climate system. That effect will not go away until our pollution and, as I'll discuss in further detail in a future post, our land use changes stop.
A great deal of discussion in the global warming debate has centered around the ability of models to accurately project the state of future conditions. So let's take a quick look at how previous modeling efforts have performed with time:
Source: IPCC WGI Technical Summary Figure TS.26.
What does this graph so? A number of things. The three previous IPCC Assessment Reports' trends and ranges are the first thing to notice (colored triangles). The 2nd and 3rd AR underestimated the warming from 1995-2005 while the 1st AR captured the observed warming (black line) within its range. Were any of them perfect? No, but that's not the point of the projections. The projections are designed to give policymakers a sense of the range of future conditions that are expected. Next, note the trend lines projected by the different SRES scenarios to the right of the year 2000. The commitment line (orange) is the projected response by the climate system to the greenhouse forcing already in place, based on the assumption that GHG and aerosol concentrations are held constant at their levels from 2000 forward (i.e., no future increase or decrease). We already know that GHG and aerosol concentrations have changed since 2000. Note that the effects on the climate system by aerosols continue to be examined by research scientists and currently constitute the largest uncertainty in the radiative forcing of the climate system. It doesn't make that much difference for the purposes of this graph, but the A1FI scenario-based projections (and a few others) isn't indicated. This makes sense from the following perspective: the difference between the A1FI scenario and those included in this graph aren't very large until the 4th quarter of this century. Despite this, the problem I have with the presentation of scenarios has to do with our current emissions path:
As this graph shows, actual emissions starting from 2000 was within the small range encompassed by the SRES scenarios. By 2004 however, actual emissions were at the upper range of the SRES scenarios. The dip in 2008-2009 was due solely to the economic collapse of those years - the most polluting and most costly activities were shut down first. As economies around the world started to recover, those activities came right back online and emissions jumped by the highest year-over-year amount in history. The end result is this: emissions are closer to the A2 or A1FI scenarios than they are the A1B or B1 scenarios in the previous graph. As a follow-on to what I wrote above, the AR4 projections are holding up very well with time, as should be expected. Back to the problem I have with the previous graph: given the actual emissions history, policymakers will be better served by work that concentrates on emissions scenarios closer in nature to the A2 and A1FI scenarios.
SRES Warming Projections
The models used to project future climate conditions overall have done fairly well and have gotten better with time, even though they're missing critical feedback processes and some radiative forcing information. With that knowledge in hand, what do the SRES warming projections look like?
Source: IPCC AR4 Technical Summary Figure TS.32.
The dark lines are the means of 20th-century simulation (black) and 4 IPCC scenarios (orange - commitment; blue - B1; green - A1B; red - A2). The lighter colors surrounding the mean represent the +-1 standard deviation range of values about the respective means. The zero-reference line is the 1980-1999 base period in this graph. The number of models run per period are shown beneath the curves. This has serious implications for evaluating climate post-2100 as the commitment and A2 scenario were not run past 2100 while the B1 and A1B scenarios were run out to 2300. Both of the latter sets of simulations showed continually increasing temperatures even out to the year 2300 and likely beyond. As discussed above, our historical emissions through 2010 are more closely characterized by the A2 scenario than the B1 or A1B scenarios. As seen in the TS.32 graph, global temperatures rapidly warm through 2100 and would likely continue to do so well beyond 2100. This is the most likely path that actual global temperatures will take given the lack of action to appropriately change our influence on the climate. Moreover, I will once again take the opportunity to point out that these scenarios more likely underestimate the eventual warming and overestimate it due to the lack of inclusion of feedback processes in the model simulations. Hundreds of peer-reviewed papers have been written since the AR4 was produced that have demonstrated this to be the case.
For reasons I will discuss in future posts, we will be unable to reach the emissions reduction targets required by the B1 scenario - one of the better cases studied. However, for purposes of context, let us assume that such an achievement can and will be made. On top of the 0.74C warming over 1906 average temperatures already recorded, and additional 1.8C warming over 2000 temperatures are likely to occur under the B1 scenario. The unattainable B1 scenario means that the much-publicized target of no-more-than 2C warming is very unlikely to be met. I will have more information on that in future posts as well.
Cross-posted at WeatherDem - the blog.