I made some graphs of global temperature change according to the three major compilations based on measured surface temperatures: GISS, HadCRU and NCDC. They are expressed as the temperature difference (“anomaly”) with respect to the 1901-2000 average as the baseline.
Temperatures jiggle up and down, but the overall trend is up: The globe is warming.
To highlight the long term trend more clearly, below the same figure with in addition the 11 year running mean (which stops 5 years short of each endpoint for lack of data to calculate the mean):
Some people prefer you to only look at the last dozen of years:
Often, the last datapoint (representing 2009) is omitted, and only HadCRU temperatures (in blue) are shown, to create the most visually compelling picture for claiming that “global warming has stopped” or even reversed (“blogal cooling”, pun intended).
If however we look at the trend through the average of the three datasets over the period 1975-2009 (during which greenhouse gas forcing was the dominant driver of climate change), we see the following:
The trend over 1975 to 2009 is approximately the same (0.17 +/- 0.03 degrees per decade) for all three temperature series.
The error represents the 95% confidence interval for the trend, i.e. if you were to repeat the trend analysis a hundred times on the real underlying data, 95 times you would find that the trend is within the range 0.14 to 0.20 degrees per decade.
The thin black lines represent the 95% confidence “predictions bands” for the data: Based on the observed variability, 95% of the data are expected to fall within these lines.
The observed yearly variability in global temperatures (sometimes exceeding 0.2 degrees) is such that 10 years is too short to discern the underlying long term trend (0.17 degrees per decade). There is no sign that the warming trend of the past 35 years has recently stopped or reversed.
A major difference between the datasets is that HadCRU omits the arctic (in effect assuming that is warms as the global average), while GISS estimates it by interpolation. I don’t know about NCDC. See also RealClimate and James Hansen.
1998 was a record warm year in large part because of a very strong El Nino event. If the effect of the ENSO cycle is removed, the warming trend becomes even more apparent, see e.g. RealClimate. Other rebuttals of the spurious 1998-claim at SkepticalScience, Coby Beck, Zeke Hausfather, RealClimate, Scott Mandia, Greenfyre (including lots more links) and Peter Sinclair of the denial crock of the week youtube videoseries.
Four independent statisticians were given the data (up to 2008) and asked to look for trends, without being told what the numbers represented. Not surprisingly, they found no evidence of a downward trend. Story retold e.g. here and here.
Robert Grumbine explains the art of cherrypicking and why it is not science.
Update: If you want higher resolution versions of any of the figures here you can email me via the link on the right (under “pages”).