Today, President Barack Obama unveiled his plan to ration carbon, and boost the development and adoption of renewable energy technologies as a way to combat the looming threat of catastrophic climate change. Perhaps the president is anxious to proceed because he thinks that Robert Weissman, the head of the activist group Public Citizen is right when, in a press release in response to the president's speech, he declares:
Catastrophic climate change poses a near-existential threat to humanity. We need a national mobilization – and indeed a worldwide mobilization – to transform rapidly from our fossil fuel-reliant past and present to a clean energy future. We need a sense of urgency – indeed, emergency – massive investments, tough and specific standards and binding rules.
Weismann, however, asserts that the president's plan is "a day late, and a dollar short." Of course, the president is relying on the projections made by computer climate models that show significant warming in response to ever increasing levels of carbon dioxide produced by burning fossil fuels. So it would really be interesting to see how the models compare to actual global average temperature trends. As it happens, University of Alabama in Huntsville climatologist John Christy has done just such a comparison. The Coupled Model Intercomparison Project 5 (CMIP5) aims to
- evaluate how realistic the models are in simulating the recent past,
- provide projections of future climate change on two time scales, near term (out to about 2035) and long term (out to 2100 and beyond), and
- understand some of the factors responsible for differences in model projections, including quantifying some key feedbacks such as those involving clouds and the carbon cycle.
In an effort to see how well the model projections match actual temperature trends as measured by satellites and weather balloons, Christy ran accessed (in response to a query from me) 73 of the models involved in CMIP5 from 1979 (when the satellite data starts) through 2025. Christy uses the representative concentration pathway 8.5 (RCP 8.5) in his computer runs analysis, noting that the various emissions scenarios only diverge after 2030, so when comparing with observations (i.e. through 2012), it doesn't matter which scenario (2.6, 4.5, 6.0, 8.5), they all have the same forcing through 2012. See results below:
The graphic above depicts the global lower troposphere temperature projections from 73 CMIP5 models from 1979 to 2025 compared to an average of the satellite data from UAH and RSS (blue boxes) and weather balloons (green circles) for the global lower troposphere temperatures since 1979 until now. Note nearly all the model runs project much warmer temperatures than the globe has recently experienced. The thick black line is the average projection of the 73 models.
Christy's calculations are consistent with some recent good news with regard to climate sensitivity that suggests that humanity may have the luxury of more time to address whatever problems man-made global warming may pose. As I reported last month, researchers in a study published in Nature Geosciences have estimated significantly lower equilibrium climate sensitivity and transient climate response numbers. Climate sensitivity is conventionally defined as the amount of eventual warming that would occur if carbon dioxide concentrations in the atmosphere are doubled and transient climate response is the actual temperature rise the planet will have experienced at the time this doubling happens.
From the study:
The most likely value of equilibrium climate sensitivity based on the energy budget of the most recent decade is 2.0 °C, with a 5–95% confidence interval of 1.2–3.9 °C. …
The best estimate of TCR based on observations of the most recent decade is 1.3 °C (0.9–2.0 °C).
The new climate sensitivity estimate (and a slew of others) is well below (1 degree centigrade) the U.N. Intergovernmental Panel on Climate Change's old estimate of climate sensitivity of between 2 to 4.5 degrees Celsius with a best estimate of 3 degrees. The upshot is that these lower estimates give humanity, as the popular science journal New Scientist explained, "a second chance to save the climate." In other words, draconian measures to reduce carbon emissions from burning fossil fuels may not be necessary to prevent the worst outcomes (whatever they would be) of future man-made global warming. So again, are you really sure that we need to rush into carbon rationing, Mr. President?
Heads up: I will be doing an analysis of the specific economic effects of President Obama's carbon rationing proposals later this week.