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Updated Physical Science Basis of Climate Change

IPCC Fifth Assessment Report (2013)

On September 27, the Intergovernmental Panel on Climate Change (IPCC) published a Summary for Policy Makers that assesses the current status of climate change science. More than 200 scientists in the panel's Working Group I contributed to the report, titled Climate Change 2013: The Physical Science Basis; the full report is scheduled for release later this week. It is the first part of the IPCC's Fifth Assessment Report on Climate Change (the first assessment was conducted in 1989).

The IPCC is considered to be the global benchmark in our understanding of climate change. The summary reflects the work of 209 climate scientists, with input from thousands of additional government officials and experts; it draws on 9,200 peer-reviewed studies and publications regarding climate change and has the approval of 195 governments. Reports by two additional working groups on climate change impacts, adaptation, vulnerability, and mitigation are expected next year.
The report on the physical science of climate change contains detailed information on observed and projected changes to the atmosphere, oceans, poles, soils and rocks due to climate change. Its observations are more definite than previous reports.

Observations of a Changing Climate

  • Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.

  • Each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850. In the Northern Hemisphere, 1983-2012 was likely the warmest 30-year period of the last 1400 years.

  • Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90 percent of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971.

  • The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years. CO2 concentrations have increased by 40 percent since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. The ocean has absorbed about 30 percent of the emitted anthropogenic carbon dioxide, causing ocean acidification.

Conclusions and Projections

  • IPCC is now 95 percent certain that humans are the source of the rapid warming of the planet since 1950. This is the highest level of certainty ever attributed by scientists, experts and governments to the role of human activities in climate change.

  • By the end of the century, warming will likely reach at least 3.6 degrees Fahrenheit (2 degrees Celsius), a ceiling internationally agreed upon as necessary to avoid the worst effects of climate change. This warming will lead to increased ice sheet melting, sea level rise, droughts and heat waves, additional ocean acidification as well as changes to precipitation amounts and patterns.

  • It is virtually certain that global mean sea level rise will continue beyond 2100, caused by thermal expansion of sea water and made worse by sustained (possibly even irreversible) loss of mass from ice sheets.

  • For the first time, the IPCC defines a global carbon "budget" that should keep warming to below 2 degrees Celsius. The budget allows for cumulative (total) anthropogenic emissions of approximately a trillion metric tons of carbon. Human-caused emissions since the beginning of the Industrial Revolution already add up to more than half of the budgeted trillion tons.

  • It is the total sum of all human-caused carbon emissions that will determine the nature and intensity of climate change impacts. Because cumulative total emissions of CO2 and global mean surface temperature response rise at roughly the same rate, higher cumulative CO2 emissions will result in more warming and a lower warming target (or a higher likelihood of remaining below a specified warming target) will require lower cumulative CO2 emissions.

The Summary for Policymakers and the full Physical Science Basis report for the IPCC Fifth Assessment Report are available on the IPCC website