Observed temperature changes
Annual (thin lines) and five-year lowess smooth
(thick lines) for the temperature anomalies averaged over the Earth's land area (red line) and sea surface temperature anomalies (blue line) averaged over the part of the ocean that is free of ice at all times (open ocean).
Multiple independently produced datasets confirm that between 1880 and 2012, the global average (land and ocean) surface temperature increased by 0.85 [0.65 to 1.06] °C. Since 1979 the rate of warming has approximately doubled (±0.03 °C per decade, against 0.13±0.02 °C per decade). 0.07 Climate proxies show the temperature to have been relatively stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age.
Although the increase of the average near-surface atmospheric temperature is commonly used to track global warming, over 90% of the additional energy stored in the climate system over the last 50 years has accumulated in the oceans. The rest has melted ice and warmed the continents and the atmosphere.[c]
The warming evident in the instrumental temperature record is consistent with a wide range of observations, as documented by many independent scientific groups. Examples include sea level rise, widespread melting of snow and land ice, increased heat content of the oceans, increased humidity, and the earlier timing of spring events, e.g., the flowering of plants.
Global warming refers to global averages, with the amount of warming varying by region. Since 1979, global average land temperatures have increased about twice as fast as global average ocean temperatures. This is due to the larger heat capacity of the oceans and because oceans lose more heat by evaporation. Where greenhouse gas emissions occur does not impact the location of warming because the major greenhouse gases persist long enough to diffuse across the planet, although localized black carbon deposits on snow and ice do contribute to Arctic warming.
The Northern Hemisphere and North Pole have heated much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, its arrangement around the Arctic Ocean has resulted in the maximum surface area flipping from reflective snow and ice cover to ocean and land surfaces that absorb more sunlight. Arctic temperatures have increased and are predicted to continue to increase during this century at over twice the rate of the rest of the world. As the temperature difference between the Arctic and the equator decreases, ocean currents like the gulf stream that are driven by that temperature difference are weakening. Studies have also linked the rapidly warming Arctic to extreme weather in mid-latitudes as the jet stream becomes more erratic.
The rate of ice loss from glaciers and ice sheets in the Antarctic is a key area of uncertainty since Antarctica contains 90% of potential sea level rise. Polar amplification and increased ocean warmth are undermining and threatening to unplug Antarctic glacier outlets, potentially resulting in more rapid sea level rise. To date, increased snowfall in Antarctica has offset a third of ice loss from West Antarctica, with East Antarctica ice sheets recently beginning to shed mass as well.
Short-term fluctuations vs. overall trend
Because the climate system has large thermal inertia, it can take centuries for the climate to fully adjust. While record-breaking years attract considerable public interest, individual years are less significant than the overall trend. Global surface temperature is subject to short-term fluctuations that overlay long-term trends, and can temporarily mask or magnify them.
An example of such an episode is the slower rate of surface temperature increase from 1998 to 2012, which was dubbed the global warming hiatus by the media and some scientists. Throughout this period ocean heat storage continued to progress steadily upwards, and in subsequent years surface temperatures have spiked upwards. Climate models account for the global warming hiatus by incorporating heating and cooling from El Niño / La Nina events, sunspot cycles, and volcanic eruptions that reach the stratosphere.