National Oceanic and Atmospheric Administration:
The greenhouse effect is unquestionably real and helps to regulate the temperature of our planet. It is essential for life on Earth and is one of Earth's natural processes.... So, the concern is not with the fact that we have a greenhouse effect, but whether human activities are leading to an enhancement of the greenhouse effect by the emission of greenhouse gases through fossil fuel combustion and deforestation.
Human activity has been increasing the concentration of greenhouse gases in the atmosphere (mostly carbon dioxide from combustion of coal, oil, and gas; plus a few other trace gases). There is no scientific debate on this point.
Global surface temperatures have increased about 0.74°C (plus or minus 0.18°C) since the late-19thcentury, and the linear trend for the past 50 years of 0.13°C (plus or minus 0.03°C) per decade is nearly twice that for the past 100 years. The warming has not been globally uniform. Some areas (including parts of the southeastern U.S. and parts of the North Atlantic) have, in fact, cooled slightly over the last century. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N. Lastly, seven of the eight warmest years on record have occurred since 2001 and the 10 warmest years have all occurred since 1995.
Global mean sea level has been rising at an average rate of 1.7 mm/year (plus or minus 0.5mm) over the past 100 years, which is significantly larger than the rate averaged over the last several thousand years. Depending on which greenhouse gas increase scenario is used (high or low) projected sea-level rise is projected to be anywhere from 0.18 (low greenhouse gas increase) to 0.59 meters for the highest greenhouse gas increase scenario.
http://www.ncdc.noaa.gov/oa/climate/globalwarming.html
From the Our Changing Planet report signed by the George W. Bush Administration's Secretaries of Commerce and Energy (as reported in New Scientist):
The document reports that global warming in the first half of the 20th century, estimated at 0.2°C above pre-industrial temperatures, "was likely due to natural climate variation", including increased solar activity.
But the approximate 0.5°C rise over the second half of the century, most pronounced in the last 30 years, can only be explained when factors related to human activity, such as carbon dioxide and other greenhouse gas emissions, are taken into account.
"There's nothing else we can blame it on, really," says Kevin Trenberth, head of the climate analysis section at the US National Center for Atmospheric Research in Boulder, Colorado, where computer simulations produced the result. "If we don't put the changes in carbon dioxide into our models, we don't get global warming out."
Thomas Graedel, an industrial ecologist at Yale University, has reviewed the US government's climate change research strategy and says the report's acknowledgment of a human influence on global warming is encouraging.
"Well over 98% of scientists competent in this area would agree with that," he told New Scientist.
Climate Change 2007: Working Group I: The Physical Science Basis -- a report by the Intergovernmental Panel on Climate Change
(Internationally, the Intergovernmental Panel on Climate Change (IPCC), under the auspices of the United Nations (UN), World Meteorological Organization (WMO), and the United Nations Environment Program (UNEP), is the most senior and authoritative body providing scientific advice to global policy makers. -- NOAA)
The simultaneous increase in energy content of all the major components of the climate system and the pattern and amplitude of warming in the different components, together with evidence that the second half of the 20th century was likely the warmest in 1.3 kyr (Chapter 6) indicate that the cause of the warming is extremely unlikely to be the result of internal processes alone. The consistency across different lines of evidence makes a strong case for a significant human influence on observed warming at the surface.
Anthropogenic change [i.e. change caused by human activity] has been detected in surface temperature with very high significance levels (less than 1% error probability). This conclusion is strengthened by detection of anthropogenic change in the upper ocean with high significance level. Upper ocean warming argues against the surface warming being due to natural internal processes. Observed change is very large relative to climate-model simulated internal variability. Surface temperature variability simulated by models is consistent with variability estimated from instrumental and palaeorecords. Main uncertainty from forcing and internal variability estimates (Sections 9.4.1.2, 9.4.1.4, 9.5.1.1, 9.3.3.2,9.7).
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch9s9-7.html
World Meteorological Association
The build-up of greenhouse gases in the atmosphere during the 20th century has resulted from the growing use of energy and expansion of the global economy. Over the century, industrial activity grew 40-fold, and the emissions of gases such as carbon dioxide (CO2) and sulphur dioxide (SO2) grew 10-fold.
http://www.wmo.int/pages/themes/climate/causes_of_global_warming.php
