Trend Analysis in the Context of Global Warming

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There are many sources ranging from politicians to mass media that claim that the Earth is actually cooling, not warming as scientists say it is. Unusually cold periods and local changes in weather are generally the cause for these statements. Why are these claims inaccurate?

Big Ideas: 
  • Global warming, as the name implies, concerns its self with an increase of the global temperature.
  • Regional changes in temperature cannot be used to reach conclusions about the global temperature.
  • Making claims about global warming which are based on short term and local data is an example of "cherry picking", an unreliable way to interpret data.

As recently as 2009, an article was published in the Globe and Mail1 calling global warming a "propagandized theory" after a very cold July in Ontario. "Where is that global cooling alert?" it asks. It is true that when viewed on a regional scale, data shows that central Canada was cold compared to the July average over the last 30 years. Other parts of the world, however, experienced a hotter July than average, and the global average made July 2009 the fifth warmest on record. Even the local temperature in Ontario three months later was much higher than the past average, and September 2009 was the second warmest on record. What can we conclude from this? A one-month-long cold snap, especially on a regional level, is surely not sufficient to issue concern over a global cooling of the planet.

Aside from claims of global cooling, others say that over the last several years the global temperature has been declining or staying constant, not rising. While looking at global temperature trends and several years worth of data are both reasonable things to do, some commentators perform an act called "cherry picking". This means portions of the data are chosen that support their claim while other portions are ignored. For example, short term variations (though possibly years in length) caused by events such as El Niño or La Niña events and volcanic eruptions can cause relatively short periods of cooling. When looking at a longer period of time, however, we see that the global temperature is actually slowly, steadily rising. The World Meteorological Organization specifies thirty years of data for finding a statistically robust trend in the global climate.

Check out this video presented by the Pacific Institute for Climate Solutions to learn more:


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