Autumn 2016    #22
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2015, the Hottest Year Ever Observed

El Niño and global warming combined to top all heat records

At top: Map of the number of "extremely warm days" in 2015 compared to the 30-year average number for the year.. Bottom: Year-to-year percentage of extremely warm days.  Credit: NOAA website; photo: Bob James via Creative Commons

     The strongest El Niño in fifty years joined forces with global warming (which returned after a 14-year “hiatus”) to cause 2015 to be the warmest year ever observed on Earth. This is a repeat performance, as 2014 was also record warm. Here we report highlights of the “State of the Climate”1 report which the American Meteorological Society publishes in August of each year; our news is from the 2015 report. The prolonged heat continued the drought conditions in several long-suffering regions, and enabled wildfires to scorch a huge area of land.

     Another way to assess whether 2015 was the hottest year is to total up the number of “extremely warm days” in a year compared to the average number per year in a 30-year period; a day is “extremely warm” if it is warmer than 90% of all historic days in that season. Emily Greenhalgh reported that the “State of the Climate” used averages from 1961—1990, and mapped the number of extremely warm days in the figure at left. Four large regions of extreme warmth stand out: all of western North America; all of Europe; much of central Asia including most of China; and Australia. In the graph below the map, the percentage of extremely warm days reached an all-time high in 2015.

    There were two other climate milestones last year. The concentration of carbon dioxide (CO2) reached 400 parts per million parts (ppm) of air in the annual average—for the first time in the last 800,000 years, it is believed—much longer than our species Homo sapiens has walked on the Earth. And for the first time, the global annual average temperature was 1ºC higher than the average in the middle-to-late 19th century. The climate of that century is considered typical of pre-industrial times.

    El Niño is always associated with drought in southeast Asia and other well-defined regions. A major drought in Indonesia enabled growth of major fires that, amazingly, ignited layers of combustible peat below the soil surface; the burning peat added even more CO2 to the atmosphere.

(continues below, at left)

    Many fires were not wildfires; they were set by humans to clear debris or open up new land for agricultural cultivation. This is often called “biomass burning.” In tropical Asia last year, biomass burning emitted three times the usual amount of CO2. The two islands of Sumatra and Kalimantan in Indonesia were responsible for the lion's share of these carbon emissions.

    North America was another region where biomass burning was extensive: about 50% greater than the average in the period 2001—2014.

    Some regions received excessive rainfall, especially the equatorial Pacific (where excessive rain is expected during an El Niño), the Gulf of Mexico, and South America.

    Pakistan withered under the most severe heat wave since 1980, in June of last year. Over 1000 people lost their lives in Karachi because of the heat.

    The record-breaking warmth led to some macabre happenings that may disturb those who watch their health. On 24 August 2016, the Guardian newspaper reported,
         “Record-high temperatures melted Arctic permafrost and released deadly anthrax spores from a thawing carcass of a caribou that had been infected 75 years ago and had stayed frozen in limbo until now. This all suggests that it may not be easy to predict which populations will be most vulnerable to the health impacts of climate change.”

      Climate in the United States in 2015

    Last year was the second warmest and third wettest year in the historical record dating from 1895 in the United States. In the early months of 2015, six western states experienced record warmth, while 24 eastern states suffered extreme cold, and Boston and Chicago witnessed record-setting cold. Texas and Oklahoma were drenched by record-setting yearly precipitation, which ended a drought that began in 2010. Despite the wet year for the nation as a whole, drought took a firm hold in the West, largely because the 2014—15 winter had been so warm that much precipitation fell as rain, rather than as snow, in the mountains. Snowpack accumulated to a depth far below normal.

    The remarkably thin snowpack in 2014—15 in the mountains of the Pacific coastal states caused extremely low water levels in reservoirs, and hence a water shortage. The associated drought cost the economy of California some $2.7 billion dollars in 2015, according to Doyle Rice of USA Today ( ). Agriculture took 84% of that hit. The parched landscape enabled a record number of wildfires to burn.


1. "State of the Climate in 2015"- J. Blunden and D. S. Arndt, Editors., (2016): special supplement to the Bulletin of the American Meteorological Society, Aug. 2016, vol. 97, no. 8, doi:10.1175/2016BAMSStateoftheClimate.

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