This analysis focuses on global temperature anomaly trends for the satellite era beginning in 1979. Prior to this time, data coverage over oceans was much more sparse and this problem is worse moving farther back in time. Since the oceans cover 71 percent of the earth’s surface, poor coverage over the oceans leads to great uncertainty in global temperature anomaly estimates and trends prior to the satellite era. Consequently, I have little confidence in any global temperature trend analyses that include estimates from before the satellite era.
As mentioned in my previous post, there are three main relatively independent sources of global surface temperature anomaly estimates available, derived from: the Global Historical Climate Network (and sometimes including additional land stations) coupled with sea surface temperature measurements (GHCN), global weather forecast model input data, and satellite estimates of lower tropospheric temperatures. There are multiple groups that compile and publish estimates from these sources but for simplicity, estimates from four of these groups are presented here. For the GHCN related estimates, this analysis uses the US National Center for Environmental Information (NCEI) estimates and the Berkeley Earth Surface Temperature (BEST) estimates. These are compared to the satellite estimates from the University of Alabama Huntsville (UAH) and global forecast system (GFS) estimates provided by the University of Maine (UM) Climate Change Institute (CCI).
I compiled and graphed monthly global temperature anomaly estimates from each of these four groups for the period 1979 through September 2015 and calculated linear regression trends to indicate the overall change over this period. The UAH satellite estimates in Figure 1 show the lowest trend at +0.0009 degrees Celsius (C) per month over the period of nearly 37 years, which corresponds to +1.08C per century if maintained. The next lowest trend, shown in Figure 2, is +0.0011C per month from the UM CCI GFS estimates, which corresponds to +1.32C per century if maintained. The second highest trend is +0.0013C per month for the NCEI GHCN estimates shown in Figure 3, which corresponds to +1.56C per century. The highest trend of +0.0014C per month is from the BEST GHCN estimates shown in Figure 4 and corresponds to +1.68C per century if maintained.
Figure 1. Global temperature anomaly trend for 1979-2015 based on UAH satellite monthly estimates.
Figure 2. Global temperature anomaly trend for 1979-2015 based on UM CCI GFS monthly estimates.
Figure 3. Global temperature anomaly trend for 1979-2015 based on NCEI GHCN monthly estimates.
Figure 4. Global temperature anomaly trend for 1979-2015 based on BEST GHCN monthly estimates.
My best guesstimate of uncertainty for global temperature anomaly estimates is at least plus or minus 0.3C to 0.5C and thus these trends appear to be significant, although with a fairly low confidence. I suspect that the satellite derived estimate of +1.08C per century is the most accurate of these estimates, followed by the GFS estimate of +1.32C per century, neither of which is especially alarming considering evidence of much larger century scale changes in temperatures in the past during our current Holocene epoch as previously described in this blog. Also, we have no way of knowing whether these trends will be maintained for a full century and so far for the first 15 years of the 21st century, there is evidence that the upward trend could be ending or at least slowing. My next post will take a closer look at global temperature trends for the 21st century so far.