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Comparing recent observations to 2001 IPCC report projections

May 11th, 2007 by Hasenkopf · 4 Comments

Just a quick note this week: A Brevia article by Rahmstorm et al. appeared in Science last Friday that compared climate model projections from the 2001 Assessment report of the IPCC with recent climate observations. Specifically, the article compared the 2001 model projections* to observations of global mean CO2 concentration, global mean air temperature (land and sea), and global mean sea level change from 1990 to the present. Here were their results:


Figure Caption: Changes in key global climate parameters since 1973, compared with the scenarios of the IPCC (shown as dashed lines and gray ranges). (Top) Monthly carbon dioxide concentration and its trend line at Mauna Loa, Hawaii (blue), up to January 2007, from Scripps in collaboration with NOAA. ppm, parts per million. (Middle) Annual global-mean land and ocean combined surface temperature from GISS (red) and the Hadley Centre/Climatic Research Unit (blue) up to 2006, with their trends. (Bottom) Sea-level data based primarily on tide gauges (annual, red) and from satellite altimeter (3-month data spacing, blue, up to mid-2006) and their trends. All trends are nonlinear trend lines and are computed with an embedding period of 11 years and a minimum roughness criterion at the end, except for the satellite altimeter where a linear trend was used because of the shortness of the series. For temperature and sea level, data are shown as deviations from the trend line value in 1990, the base year of the IPCC scenarios.

The CO2 concentrations obviously match the projections best, while both temperature and sea level changes were underestimated by the IPCC 2001 report projections. The fact that the global mean air temperature and sea level change do not match as closely as the CO2 one makes sense since the other two are feedbacks of CO2 concentrations as well as other GHGs and aerosols, and therefore models would depend on how accurately we can project enhancement or depletion rates of all of those things and how well we understand the climate’s sensitivity to their concentrations. As for why the actual temperature and sea level change both consistently increase at rates higher than the models project, it is hard to say whether this is caused by intrinsic variability, an unaccounted for decrease in aerosol cooling, or an underestimation in models of the climate’s sensitivity to CO2.

I wonder what the latest IPCC climate models would predict for the same climate change parameters if they were run for projections over the same period. Could they better predict the observations? Any current-day Earth climate modelers or people up on the latest IPCC report know?

*As the article states, the 2001 IPCC scenarios are essentially independent ofrom the observed climate data since 1990. Additionally, global sea level data was not available at that time.

Tags: climate

4 responses so far ↓

  • 1 seand // May 11, 2007 at 7:57 am


    I’m a little confused by the sea level graph. The units say cm, but that seems incredibly high, given that pre-industrial to present sea level changes were on the order of mm (at least, I thought..). Can you clarify this? Also, the new IPCC is out. I haven’t looked yet, but they may have included this recent data in their ‘hindcast’ of 20th century climate.

  • 2 seok // May 11, 2007 at 9:03 am

    Someone needs to explain to me about the sea level change plot. The units in cm makes it look incredible. In the Science article, the authors wrote 3.3 mm/year from 1996-2006. That makes it 33 mm (3 cm) over 10 years. Maybe it is that extreme.

  • 3 Christa // May 11, 2007 at 9:38 am

    Hi Sean and Brian,

    I didn’t have an initial feel for these numbers, however, in the latest IPCC report- in section 5.5 FAQ 5.1, Figure 5.13, 5.14 confirm the scale of the figure in the article and also the current observed sea level rising rate of ~3mm/year, at least at my glancing view. Go here to see the section I am talking about: http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Ch05.pdf

    In section it states: “Numerous papers on the altimetry results (see Cazenave and Nerem, 2004, for a review) show a current rate of sea level rise of 3.1 ± 0.7 mm yr–1 over 1993 to 2003 (Cazenave and Nerem, 2004; Leuliette et al., 2004; Figure 5.14).”

    This corresponds to a ~3cm/decade rate of rise, which matches with the 1990-2000 section of the figure in the article.

    So I feel pretty sure the figure, though disconcerting, is right-…if anyone discovers otherwise, please let me know!


  • 4 Hank Roberts // Jun 12, 2007 at 4:22 pm

    Typo in the lead author’s name.
    And thanks for finding the article!

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