The Instrumental temperature record To quantify any changes in climate it is crucial to have a good record of climate using scientific instruments. Such a record exists from about the 1850’s, and the closer we get to the present day, the more confident we can be in the data going into this record. A number of scientific institutes maintain databases of temperature observations, and a lot of time is spent quality-controlling these datasets and ensuring they present the best possible representation of the observed temperature. Figure 1 below shows a time-series of global mean temperature from the records maintained by 3 major climate research centres. source: imgur Figure 1. Time-series of global mean near-surface air temperature from 3 different climate research centres. The black line shows the dataset maintained by the Met Office Hadley Centre and Climate Research Unit at UEA. The grey shading shows the 95% confidence interval on this time-series. The red line shows the data from the US National Oceanographic and Atmospheric Administration (NOAA National Climate Data Center and the blue line shows data from NASA’s Goddard Institute for Space Studies. Data supplied by the Met Office © British Crown copyright 2013, the Met Office.) At the global scale we see that the global mean temperature has increased by about 0.9˚C since the start of the 20th century to the present day. This warming can be explained by an increase in greenhouse gases and there is a consensus amongst scientists that man-made emissions of these gases are the main cause of the warming. However, the temperature increase has been happening at a rate which is not uniform. During the first half of the century there was a warming of about 0.4˚C, but through the middle part of the century the global temperature stabilised and even cooled somewhat. The warming trend recommenced in the 1960’s and continued until about 2000. Since then there has been another hiatus in the warming trend, with little or no increase in the past decade. The significance of this hiatus has been a hot topic of debate in the climate science community. Given that the levels of greenhouse gases in the atmosphere have continued to increase throughout this period, we need to understand why this has not been reflected in continuous warming of the atmosphere. The latest research suggests that much of the ‘missing’ heat has been going into the oceans, which because of their high heat capacity can absorb a lot of heat without exhibiting a large increase in temperature. To verify this theory we need good observations of the temperature of the oceans throughout their depth, not just at the surface. Such observations exist but they are by no means as comprehensive as our measurements of the ocean surface temperature and so there is still a lot to learn about where the oceans are warming most, whether the atmospheric warming will accelerate again and on what timescale. One really important thing to note in the global temperature record is the presence of quite high frequency (year-to-year) variations. These fluctuations are in large part well understood. Large volcanic eruptions, particularly in the Tropics, release large quantities of tiny sulphate particles into the atmosphere which act to reflect sunlight back out to space. These can stay in the atmosphere for 1-2 years and can result in a marked global cooling during that period. Such eruptions account for some of the more marked downward spikes in global mean temperature. Some of the large upward spikes in temperature are associated with El Niño events, when a strong warming of the surface of the tropical Pacific Ocean releases huge quantities of heat into the atmosphere resulting in a temporary warming of the global mean temperature which lasts for about 1 year.
Posted on: Sun, 23 Nov 2014 21:03:54 +0000
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