365 Days of Climate Awareness 78 - Global Mean Surface Temperature

Second to atmospheric carbon dioxide concentration, global mean surface temperature (GMST) is
the fundamental measure of global warming. It's not an easy number to produce, since temperatures differ so widely across the planet surface, based on latitude, altitude, topography and weather patterns, among other influences. Temperatures are sampled from the atmosphere and ocean surface around the world. The individual records are processed separately and then averaged into one global total.

There are effects both natural and man-made on temperature records. The difference between the warmest and coldest parts of the planet can be well over 55 deg C (100 deg F). Daily and seasonal swings on the order of 50 deg C (28 deg F) occur. Cities also produce heat islands, where the collection of energy consumption and humans in close proximity can produce heat bubbles 6 deg C (3 deg F) or more higher than their surroundings.

One solution is to avoid sampling in urban areas. When it comes to seasonal variations, a common solution is to average month by month, treating each calendar month as a unit (despite their differences in length). Many multi-annual records are composed of a series of averages from one selected month. Research groups from around the world analyze their records independently and submit them to scientific bodies like the IPCC for inclusion in the larger dataset.

Since the temperature trend for the planet is our focus, the records are converted to anomalies, the difference between an individual record and a historical baseline. This avoids the obvious  impossibilities in averaging tropical with polar, and atmospheric with oceanic, temperatures. Baselines are arbitrary, but some common ones are the 1850-1900 average, the 1900 average, and the 1950 average. Temperature anomalies in recent years are all well above the baseline: the anomalies are positive and increasing.

The 2015 Paris Agreement provides a goal of keeping global temperature rise below 2, and preferably 1.5, deg C above pre-industrial levels, meaning the 1850-1900 average baseline. Climatic effects modeled to happen above the 1.5 degree anomaly are already starting to happen,however.

Tomorrow: the Paris Agreement.

Be brave, and be well.