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Isaac Held's Blog

9. Summer is warmer than winter

Black: Climatological seasonal cycle of temperature in Minneapolis  (two years are shown for clarity).  Averaging (Tmax +Tmin)/2 over >100 years for each calendar day.  Data kindly provided by Charles Fisk.  Red: Fit with annual mean plus fundamental annual harmonic

Two common questions that I (and many others) often get are “How can you predict anything about  the state of the atmosphere 100 years from now when you can’t predict the weather 10 days in advance?” and “How do you know that the climate system isn’t far more complicated than you realize or can possibly model?”  I often start my answer in both cases with the title of this post.  It may sound like I am being facetious, but I’m not; the fact that summer is warmer than winter is an excellent starting point when addressing both of these questions.

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8. The recalcitrant component of global warming

Evolution of global mean near-surface air temperature in GFDL’s CM2.1 climate model in simulations designed to separate the fast and slow components of the climate response in simulations of future climate change, as described in Held et al, 2010.

Continuing our discussion of transient climate responses, I want to introduce a simple way of probing the relative importance of fast and slow responses in a climate model, by defining the recalcitrant component of global warming, effectively the surface manifestation of changes in the state of the deep ocean.

The black curve in this figure is the evolution of global mean surface air temperature  in a simulation of the 1860-2000 period produced by our CM2.1 model, forced  primarily by changing the well-mixed greenhouse gases, aerosols, and volcanoes.  Everything is an anomaly from a control simulation.  (This model does not predict the CO2 or aerosol concentrations from emissions, but simply prescribes these concentrations as a function of time.) The blue curve picks up from this run, using the SRES A1B scenario for the forcing agents until 2100 and then holds these fixed after 2100.  In particular, CO2 is assumed to approximately double over the 21st century, and the concentration  reached at 2100 (about 720ppm) is held fixed thereafter.  The red curves are the result of abruptly returning to pre-industrial (1860) forcing at different times (2000, 2100, 2200, 2300) and then integrating for 100 years.  The thin black line connects the temperatures from these four runs averaged over years 10-30 after the abrupt turn-off of the radiative forcing.

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7. Why focus so much on global mean temperature?

Upper panel: Interdecadal component of annual mean temperature changes relative to 1890–1909. Lower panel: Area-mean (22.5°S to 67.5°N) temperature change (black) and its interdecadal component (red).  Based on the methodology in Schneider and Held, 2001 and HadCRUT3v temperatures.  More info about the figure.

Perhaps the first thing one notices when exposed to discussions of climate change is how much emphasis is placed on a single time series, the globally averaged surface temperature. This is more the case in popular and semi-popular discussions than in the scientific literature itself, but even in the latter it still plays a significant role.  Why such an emphasis on the global mean?

Two of the most common explanations involve 1) the connection between the global mean surface temperature and the energy balance of the Earth, and 2) the reduction in noise that results from global averaging.  I’ll consider each of these rationales in turn. Read the rest of this entry »