Can Climate Change be Noticed Locally?
The evidence from weather stations and ocean buoys shows unequivocally that global climate is changing, with more than 2°F warming since the late 19th Century. This is about 10 times larger than the year-to-year variability. However, at the regional and local level, the internal, natural variations in temperatures and rainfall make it harder to detect these trends.
Back in early 1998, Dr. Jim Hansen and colleagues proposed a Common Sense Climate Index (CSCI) to address this question. What would it take for someone who had lived a few decades in a particular location to sense that the local climate had indeed changed?
The basic idea was to construct an index from quantities that are familiar to the general public and readily observable, and introduce a scale that objectively indicates the significance of the change by comparing it to the natural variability of the observed quantity. Details were presented in a Discussion of the Climate Index and Climate Index Scale, as well as in a science feature story. This index is positive for warm years, negative for cool years. For an individual station and in a stable climate it would exceed the value 1 about three or four times in a 30-year period. A value higher than 2 might occur maybe once every 40 years. The paper contended that a persistent climate index of +1 or greater represented a noticeable climate change, even though people's perceptions of change are a sociological matter.
Such an index however does not capture other important effects of climate change, such as rising sea level, more intense rainfall, impacts on water resources, sea ice and snow loss, etc.
In a website accompanying the CSCI paper, the concept was illustrated using the example of New York City. Four heat related indices were shown together with the resulting combined index. They all showed a very similar behavior, and it turned out that this is generally the case. Hence, it was subsequently decided to concentrate on the simplest of those indices, the index based on seasonal temperature anomalies.
Here is what this simpler index looked like based on the unadjusted data up to 1996 (homogeneity adjustments were introduced about 10 years after the creation of that analysis):
After nearly 20 years, we have integrated the original calculations with the latest GISTEMP data. Revisiting the example of New York City, we obtain the following up-to-date CSCI graph:
Remember that an index value of 1 sustained over a few years should be enough for long-term residents to really feel a change in the temperatures. Looking at the example of New York City, we can see that this was not yet the case in 1997 when the CSCI paper was written. However, in the 19 years since then, the index has started to more frequently exceed 1 in individual years, but not quite yet in the 5-year mean.
The CSCI averaged over the contiguous United States is similar; the noise is somewhat reduced but not enough to show a significantly different pattern:
At the global scale, as is evident also from the standard GISTEMP results, the climate changes are obvious and are increasingly so.
If we look at the geographic pattern for the mean over the last five years (i.e., 2012-2016) we can see that while there is some regional variation, there are many areas where the index has greatly exceeded 1.
In particular, the tropics, while absolute changes have been smaller than in the high northern latitudes, have had bigger shifts in the CSCI because the internal/seasonal variability is much lower. But also in many areas outside the tropics the climate index has recently exceeded 1 for several years in a row, e.g. in the Western part of the US including Alaska, in Europe, in Siberia, and most spectacularly in the Arctic (e.g. Greenland).
In general, as was suggested near the end of the 1998 feature story, the areas with noticeable warmth did expand considerably, and many more people worldwide now are aware that climate change is occurring.
We intend to maintain this way of looking at the data going forward and welcome feedback on this presentation.
Hansen, J., et al. 1997. Forcings and chaos in interannual to decadal climate change. J. Geophys. Res. 102, 25679-25720, doi:10.1029/97JD01495.
Hansen, J., M. Sato, J. Glascoe and R. Ruedy 1998. Common sense climate index: Is climate changing noticeably? Proc. Natl. Acad. Sci. 95, 4113-4120.
Science Brief, March 1998: A Common Sense Climate Index: Is Climate Changing Noticeably?
For more information on NASA GISS's Common Sense Climate Index, visit: data.giss.nasa.gov/csci.
Leslie McCarthy, NASA Goddard Institute for Space Studies, New York, N.Y., 212-678-5507, firstname.lastname@example.org