NCCSP

CLIMATE CHANGE IN THE 20th AND 21st CENTURIES

Public debates continue to rage around water coolers and in the halls of Congress about the science behind climate change projections.   But, within the scientific community the question was settled long ago.  

A wide range of global changes already have occurred and begun to alter natural systems and human communities worldwide. 

Climate Change Is Here Already

Climate change isn’t coming.  It’s here. The rise in atmospheric greenhouse gases already has led to:

• a 0.75°C increase in global average temperature over the past century (NRC 2006)

• changes in seasonal timing of precipitation, shifts from snow to rain, earlier onset of snowmelt, and increased frequency of very heavy precipitation events (USGCRP 2009)

• a 0.1°C increase in ocean temperature (sea surface to 700 m) since 1961

• an average of 1.7 mm/yr rise in sea level during the 20th century (IPCC 2007; Domingues et al. 2008)

• a decline in arctic sea ice since 1978, with a steepened rate of decline since 2000 (Earth Observatory 2009)

• a change in ocean biogeochemistry, including an increase in total inorganiz carbon content, and a cumulative reduction of 0.1 pH units during the past 200 years (Haugen and Drange 1996)

Further Change Is Coming

Concentrations of atmospheric carbon dioxide currently are rising at a rate of about 2 ppm/yr, a rate that is likely to increase if current emission trajectories remain unchanged.  By the end of this century, carbon dioxide concentrations could reach levels two to three times those of the last 800,000 years.

Projections for the coming century include:

• Increase in global average surface temperatures of 2 to 4.5°C with a doubling of atmospheric carbon dioxide, and a substantially higher increase with a tripling (IPCC 2007)
• Rise in sea level of 1 to 3 m due to thermal expansion ofo oceans and glacier melt, and the possibility of a more significant rise if ice sheets break up (ACIA 2004; NRC 2002; IPCC 2007)
• More extreme storm events, with the heaviest downpours becoming heavier and the lightest becoming lighter
• Wildfire patterns will respond to temperature and precipitation changes, making some areas more susceptible to outbreaks (Krawchuk et al. 2009)
  

Although scientists from around the world agree on the underlying causes and direction of change, their work continues to tease out the details needed to anticipate how we and the world we live in will be affected in 50 to 100 years.

Three recent reports provide comprehensive summaries of current scientific knowledge:

• Climate Change Assessment Reports (Intergovernmental Panel on Climate Change 2007)
  

• Human Impact Report -  Climate Change: The Anatomy of a Silent Crisis (Global Humanitarian Forum 2009)
  

• 2009 Global Climate Change Impacts in the United States (US Global Change Research Program 2009)