Warming or not?

Posted 8/21/12

With regard to the question as to whether global warming is currently occurring, discussed in these pages in an editorial on September 24 and a letter to the editor in last week’s newspaper, we …

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Warming or not?

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With regard to the question as to whether global warming is currently occurring, discussed in these pages in an editorial on September 24 and a letter to the editor in last week’s newspaper, we offer this chart and some related verbiage from the National Aeronautics and Space Administration (NASA) website.

The chart does not show absolute temperatures; it shows the difference between the mean annual temperatures in each year compared to temperatures in a standard base period, aka “temperature anomalies.” The base period selected is 1951-1980 (see FAQs from the site quoted below). The numbers on the chart measure a combination of both land and water temperatures. According to the World Meteorological Association, more than 90% of the excess energy trapped by greenhouse gases is stored in the oceans, so both land and water must be measured to get an accurate indication of temperature changes.

Note that although there has not been a sharp increase since 1998, the mean temperature anomaly was higher in both 2005 and 2009 than in 1998. Note also that throughout the period, temperatures remained well above the base-period mean (shown as the 0 line on the chart).

This is not the first period on the chart during which temperatures could have been argued to have been moving sideways for more than 10 years. The peak of the mean annual temperature in 1944, for instance, was not exceeded until 1973.

A World Meteorological Organization report issued in March of 2014 gives the following additional information:

“Thirteen of the 14 warmest years on record have all occurred in the 21st century, and each of the last three decades has been warmer than the previous one, culminating with 2001-2010 as the warmest decade on record. The average global land and ocean surface temperature in 2013 was 14.5°C (58.1°F)—0.50°C (0.90°F) above the 1961–1990 average and 0.03°C (0.05°F) higher than the 2001–2010 decadal average.”

FAQ from the National Aeronautics and Space Administration website:

Q. What are temperature anomalies?

A. Temperature anomalies indicate how much warmer or colder it is than normal for a particular place and time. For the Goddard Institute for Space Studies (GISS) analysis, normal always means the average over the 30-year period 1951-1980 for that place and time of year. This base period is specific to GISS, not universal. But note that trends do not depend on the choice of the base period: If the absolute temperature at a specific location is two degrees higher than a year ago, so is the corresponding temperature anomaly, no matter what base period is selected, since the normal temperature that is subtracted is the same for both years.

Q. Why stick with the 1951-1980 base period?

A. The primary focus of the GISS analysis are long-term temperature changes over many decades and centuries, and a fixed base period makes the anomalies consistent over time.

Q. What is L-OTI, the Land-Ocean Temperature Index?

A. Weather stations reporting surface air temperatures (SATs) are positioned on land, which covers only one third of the planet; the rest is covered by oceans where SAT reports are rare. However, water temperatures (SSTs, sea surface temperatures) are available from ship and buoy reports and more recently there are also SST estimates derived from satellite data. Whereas SATs and SSTs may be very different (since air warms and cools much faster than water), their anomalies are very similar (if the water temperature is five degrees above normal, the air right above the water is also likely to be about five degrees warmer than normal). This is not true in the presence of sea ice, since in that case water temperature will stay at the freezing level. This allows us to use SST anomalies as proxies for SAT anomalies in regions without sea ice. L-OTI maps show SAT anomalies over land and sea ice, and show SST anomalies over (ice-free) water.

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