“Climate change is one of the most fundamental challenges ever to confront humanity. No issue is more fundamental to long-term global prosperity. And no issue is more essential to our survival as a species.”
- UN Secretary General BAN Ki-moon

What we know
In recent years we’ve seen an overwhelming scientific consensus that climate change is being caused by unprecedented, human generated emissions of greenhouse gases, mainly from the burning of fossil fuels. The intergovernmental Panel on Climate Change (IPCC) was founded in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to research and synthesize the research of the world’s climate scientists and researchers.

The fourth and most recent assessment report of the IPCC Working Group 1, which is charged with assesses the
physical scientific aspects of the climate system and climate change, was reviewed by more than 2,500 experts, had more than 800 contributing and 400 lead authors representing more than 130 countries. This report highlights three key findings:

• Climate change is, without doubt, occurring
• The Earth is warming
• There is a 90% probability that it is human induced

Excerpt from the IPCC Fourth Assessment Report (AR4)

“Palaeoclimatic information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 m of sea level rise.”

“Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land use change, while those of methane and nitrous oxide are primarily due to agriculture.”

“Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. This is an advance since the Third Assessment Report’s (TAR) conclusion that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations”. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.”

How we know what we know
Direct measurements

Direct temperature measurements have been taken and recorded since the early 18th century. The overall trend throughout this time has been one of rising mean global temperatures. 

Direct measurements of atmospheric carbon dioxide concentrations began in the 1950s when Dr. Charles Keeling began recording them in Mauna Loa Observatory in Hawaii. So accurate are these measurements that seasonal variation is observed.

In spite of this season variation, the overall trend since the 1950s is obvious. Concentrations of atmospheric CO2 has risen from about 315 ppm to more than 380 in the last half century.

Proxy records

Historical or recorded climate data accounts for a very small fraction of the Earth's history and therefore provide an incomplete picture of the Earth's climate history. In order to complete the picture scientists study natural phenomena that are climate dependent. 

Paleoclimatic proxy records fall into three categories - glaciological, geological, and biological. 

Ice cores are extensively studied by paleoclimatologist because they provide more climate information than any other natural recorder (e.g. tree rings and sediments). From an ice core scientists can determine age, temperature, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity and a variety of other climate indicators.

The Vostok ice core from Antarctica is more than 2 miles long and provides climate information going back about 420,000 years.  

The above timeline is from the National Ice Core Laboratory

Learn more about ice core drilling in Greenland in the video "Drilling Back to the Future: Climate Clues from Ancient Ice on Greenland" from Climate Central below. 

NEEM Ice Drilling Project from Climate Central on Vimeo.

Additional climate science resources

• Climate Change - Science. U.S. EPA 
• The Science of Climate Change: Global and U.S. Perspectives. Pew Center on Global Climate Change