The Science Behind Climate Change: Air and Land

Weather Station, Glacier National Park. Suzanna Soileau/USGS

The Intergovernmental Panel on Climate Change (IPCC) published its fifth Assessment Report in 2013-2014, synthesizing the latest science behind global climate change. Here are the highlights about atmospheric and surface observations:

Atmospheric Composition

  • The concentrations of carbon dioxide, the most common greenhouse gas, increased to 390.5 ppm in 2011; this is 40% greater than in 1750. The latest estimate (for January 2014) puts atmospheric carbon dioxide at 397.8 ppm. These measurements were obtained as part of the famous Mauna Loa Keeling dataset.

  • Atmospheric methane was 1803.2 ppb in 2011, a 150% increase since 1750. Methane increased again beginning in 2007 after a flat period from 1999 to 2006. Fast-warming Arctic regions appear to play an important role in this increase.

  • Nitrous oxide reached 324.2 ppb, a 20% increase since 1750. The increasing trend is of the same magnitude for 2005-2011 as it was for 1996-2005. Most of the increase observed appears to be due to fertilizer use in agriculture.

  • The amount of water vapor in the atmosphere appears to be little changed since 1992. However, the degree of confidence about this estimate is low.

  • The amount of airborne particles in the air has decreased over Europe and the eastern US, but increased over eastern and southern Asia. These particles originate from human activity (fossil fuel and biofuel burning), and natural sources (desert dust, sea salt, volcanoes).


  • When combining land and ocean surfaces, the mean global temperature has increased by 0.72 degrees Celsius (1.3 degrees F) between 1951 and 2012. Some places experienced large temperature increases (the Arctic, northern Asia, northern North America), but a few limited areas experienced actual temperature decreases (part of the North Atlantic offshore from Greenland).

  • The warming effect of urban heat island and of changing land use is sometimes invoked as an alternative reason for the warmer measurements obtained at monitoring stations. The IPCC reports a quantification of this warming effect, which was estimated to account for less than 10% of the observed increase in temperature.

    Water Cycle

    • In the mid-latitudes of the Northern Hemisphere, precipitation has likely increased since 1901. In the western and northeastern US, less precipitation has fallen as snow, more as rain.

    • Air humidity has increased in the 1970’s but recent evidence shows a switch in trends towards drier air.

    Extreme Events

    • The number of cold days has decreased, and the number of warm days has increased since about 1950.

    • The number of high precipitation events in North America and Europe has increased since 1950, but there is no clear evidence of a global increase in flood events.

    • The report lowers the IPCC’s confidence in global-scale observed changes in drought frequency. However, the frequency and intensity of droughts in the Mediterranean and West Africa have likely increased.

    • There is low confidence about changes in cyclone (hurricanes and typhoons) activity and other severe weather events such as thunderstorms or hail (UPDATE: new evidence for links between climate change and extreme weather events).

    Large Circulation Patterns

    • Global circulation patterns have shifted towards the poles: the tropical belt became broader, the jet streams and storm tracks shifted further north in the Northern Hemisphere, and the polar vortex contracted around the North Pole. This is particularly interesting as the polar vortex became widely publicized during the especially cold 2013-2014 winter in Midwestern and northeastern United States and Canada.

      Since the previous Assessment Report, more scientific attention and better technology allowed to increase confidence in statements about global climate change. We know with a high degree of certainty that the concentration of greenhouse gases in the atmosphere, that the planet is warming, and that large circulation patterns are changing, affecting rainfall, drought, and storm events in different ways at different locations. At the same time, some uncertainty remains about trends associated with the amount of atmospheric water vapor, and the frequency and intensity of extreme events such as droughts and hurricanes.  


      IPCC, Fifth Assessment Report. 2013. Observations: Atmosphere and Surface.