Introduction: Are Global Temperatures Rising?
In 2007, in its Fourth Assessment Report, the Intergovernmental Panel on Climate Change reported that 11 of the 12 warmest years in the instrumental record since 1850 occurred between 1995 and 2006 (IPCC 2007). The updated 100-year trend, from 1996 to 2005, of 0.74°C ± 0.18°C, was greater than the 100-year warming trend at the time of the IPCC's Third Assessment Report. (The warming trend covered in the Third Assessment Report, from 1901 to 2000, was 0.6°C ± 0.2°C.) Temperature records supporting these findings have been assembled from thousands of land and ocean observation sites covering a large, representative portion of the Earth's surface and carefully controlled for possible biases arising from station and instrument changes.
In July 2010, the NASA Goddard Institute for Space Studies reported that January-June 2010 mean surface temperatures were the warmest in 131 years (GISS 2010).
Temperature anomalies by latitude: These graphs track temperature anomalies for northern, low, and southern latitudes. The black dotted line is the annual mean and the red solid line is the 5-year mean. Graphs courtesy NASA Goddard Institute for Space Studies Surface Temperature Analysis.
Temperature anomaly map: The year 2010 was the warmest 131 years, according to a report from NASA GISS. This global map shows average surface temperature anomalies, warmer than average in yellow and red, cooler in blue, and the same as the climatological average in white. Image courtesy NASA Goddard Institute for Space Studies Surface Temperature Analysis
Temperatures vary from year to year, and also from decade to decade. These variations, however, are superimposed on a longer upward trend. The range of natural variability in global temperature seems to be about ± 0.2°C, so only after the late 1970s do global mean temperatures emerge from the noise of natural variability (Karl and Trenberth 2003). The northern high latitudes have experienced greater warming than the mid-latitudes or the southern high latitudes. This is apparent in the Temperature anomalies by latitude graphs.
In some northern regions, extreme warming has been detected. Locations in Alaska and northern Eurasia, for example, have warmed by nearly 6.0°C in the winter months since 1970 (Serreze et al. 2000). The warming is not universal; some cooling has occurred in the North Atlantic and central North Pacific and is known to be a consequence of changes in the atmospheric circulation.
In its fourth assessment report, the IPCC cited atmospheric concentrations of greenhouse gases as the causative agent in warming temperatures. The panel identified fossil fuel burning and changes in land use as the primary causes of increased carbon dioxide, and agriculture as the primary cause of increased methane and nitrous oxide. Atmospheric carbon dioxide concentrations in 2005 exceeded the natural range for this gas over the past 650,000 years. The IPCC attributed a "greater than 90 percent certainty" to scientists' assertion that higher greenhouse gas concentrations have trapped more thermal radiation and consequently warmed the planet (IPCC 2007).
Temperature record: Global temperature anomalies based on paleoclimate data (green) and the instrumental record (blue) suggest that current temperatures are the warmest in the last millennium, if not longer. Graph courtesy NASA Earth Observatory, adapted from Mann et al., 2008).
Climate forcings: Earth's temperature fluctuates naturally, driven partly by three factors: El Niño, solar variability, and volcanic aerosols. Calculations of these natural factors and human forcings, however, indicate that human activity has overwhelmed natural factors in recent decades. Graphs courtesy NASA Earth Observatory, adapted from Lean et al., 2008).
Is the Cryosphere Sending Signals About Climate Change?
The cryospheric regions, or regions where water is found in solid form, provide us with direct visual evidence of temperature changes. Unlike other substances found on Earth, ice and snow exist relatively close to their melting point and may frequently change phase from solid to liquid and back again. Consequently, consistent and prolonged warming trends should result in observable changes to Earth's cryosphere. Water changing from solid to liquid and back often results in dramatic visual changes across the landscape as various snow and ice masses shrink or grow.
What are some examples of these snow and ice masses, how do we monitor their conditions, and what do the results show?
In State of the Cryosphere, snow cover, glaciers, permafrost, sea ice, ice shelves, and the related parameter sea level are discussed. In all cases, scientists attempt to monitor both the areal extent and mass of these snow and ice bodies. Areal extent is easier to determine than mass. Various forms of remote sensing, from both aircraft and satellite, allow us to look down on surfaces at varying spatial scales and over time to determine if the snow or ice covered area is expanding or contracting. Long-term monitoring includes looking at the areal extent of snow cover and sea ice, as well as changes in area and mass of mountain glaciers. In all cases shown here, regardless of parameter or measurement method, the amount of snow and ice has been decreasing over the past several decades.
Last updated: 5 August 2010