What Do the Graphs in the Climate Change Debate Mean?

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The use of data as evidence of climate change has become an increasingly debated topic in the past decade. The current U.S. political climate has only further politicized the issue, which is likely to escalate as presidential campaigns for the 2020 election begin in 2019. President Donald Trump, who has already declared he will run for re-election, has exacerbated the debate by claiming that global warming was created by the Chinese to make their manufacturing competitive. Since then he has gone on to mock Democrat Sen. Amy Klobuchar of Minnesota for her acceptance of climate change when she announced her candidacy for presidency in a blizzard and suggested that the “polar vortex” in January was evidence against global warming.

If you have been following the debate around climate change, you will most likely have seen this graph from NASA, which charts the change in global surface temperature relative to the average global temperature. You can see that this graph has a positive correlation and shows a steady increase in temperature anomaly over time. However, what does this graph actually mean? Moreover, how does NASA calculate the temperature anomaly?

The graph illustrates the change in global surface temperature relative to the 1951-1980 average temperatures. Between 1951-1980 NASA calculated the global surface temperature using a network of weather stations to create a long-term global average temperature, for which future annual global temperatures could be compared against, otherwise known as a reference value. NASA then calculated the annual global surface temperature as usual and compared it to the long-term average global temperature or the reference value. This comparison allows NASA to assess by how much global temperature has deviated from the long-term average.

A positive value means that the average global temperature for the year was hotter than the reference value, while a negative anomaly indicates that the observed temperature was cooler than the reference value. As we can see from the chart and data file, the temperature anomaly has remained higher than in previous decades. So far, politicians such as Sen. Amy Klobuchar and others who acknowledge climate change have not found an effective way to use data like NASA’s to make the case for needed legislation.

The black line is known as a LOWESS Smoothing or Locally Weighted Scatterpoint Smoothing, a tool that allows data analysts to create a smooth line through a timeplot or scatter diagram to help the viewer see the relationship between the variables.

Each point on the scatter graph, connected by the grey line, represents how much the global temperature for a given year differed from the reference value.

A useful resource when trying to understand climate data is the National Centers for Environmental Information.

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