Recent trends in global warming
In brief
- Since the start of the 20th century, the Earth's surface has warmed by about 0.74 °C.
- Globally, the decade of 2001 to 2010 was the world's warmest decade on record, warmer than the 1990s which in turn was warmer than the 1980s—1998, 2005 and 2010 are the warmest years on record.
- In Australia, 2001 to 2010 was the warmest decade on record—each decade since the 1940s has been warmer than the preceding decade.
In detail
Near-surface air temperatures
A number of agencies measure global surface temperature. Foremost among these are NASA (US), the National Climatic Data Centre (US) and the Hadley Centre (UK) which have each produced global near-surface temperature records (Figure 1). These all show a warming from 1910 to 1940, followed by a slight cooling in the 1940s, a levelling off in the 1950s and 1960s, then another warming from the 1970s onward. Averaged over the three temperature records, the value in 2010 is 0.01 °C above the value in 2005, and 0.02 °C above 1998 (WMO, 2011). Remarkably, the warmest year (2010) occurred during a La Niña event, which normally leads to relatively cool temperatures. The differences between the data reflect slightly different data coverage and underlying variability; the Hadley Centre data shows slightly greater variation associated with El Niño and does not include temperatures from the Arctic.
The surface of the Earth has warmed by 0.74 °C (uncertainty range 0.56 to 0.92 °C) from 1906 to 2005 (IPCC, 2007). When the annual data are adjusted for short-term fluctuations due to El Niño and La Niña, the warming trend is even more obvious (Fawcett, 2008). This is a statistically significant climatic change and it is very unusual in the context of the past 1700 years (refer to Warming of the 20th century).
Figure 1: Global-average temperature anomalies (°C), relative to the period 1961 to 1990, for three datasets.
Data over the past decade provides little insight into long-term trends; the period is simply too short, so trend magnitudes are highly sensitive to the choice of start and end years. For example, there has been an insignificant warming trend over the period between 1998 and 2008 (due largely to the strong El Niño in between 1997 and 1998), but a statistically significant warming from 1999 to 2008 (Easterling and Wehner, 2009). The apparent hiatus in global warming from 1998-2008 coincides with a period of little change in the total amount of natural and anthropogenic forcings (Kaufmann et al., 2011).
Nevertheless, 2001 to 2010 is the warmest decade in the instrumental record (WMO, 2011).
Easterling and Wehner (2009) state:
Numerous websites, blogs and articles in the media have claimed that the climate is no longer warming, and is now cooling. Here we show that periods of no trend or even cooling of the globally averaged surface air temperature are found in the last 34 years of the observed record, and in climate model simulations of the 20th and 21st century forced with increasing greenhouse gases. We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer-term warming.
Cities tend to be warmer than surrounding rural areas due to the heat-storage capacity of roads and buildings. This urban-rural temperature difference is often called the heat island effect. Population growth and urbanisation can lead to increases in the heat island effect, and some people have suggested that this affects global temperature trends.
The IPCC (2007) notes that:
Urban heat island effects are real but local, and have not biased the large-scale trends. A number of recent studies indicate that effects of urbanisation and land use change on the land-based temperature record are negligible (0.006 °C per decade) as far as hemispheric—and continental—scale averages are concerned because the very real but local effects are avoided or accounted for in the data sets used. In any case, they are not present in the sea surface temperature component of the record.
Tropospheric temperatures
Satellites and radiosondes measure temperature in a slab of the lower atmosphere, called the troposphere (refer to Climate models). The data should be treated with caution. The IPCC (2007) states:
Lower-tropospheric temperatures have slightly greater warming rates than those at the surface over the period 1958 to 2005. The radiosonde record is markedly less spatially complete than the surface record and increasing evidence suggests that it is very likely that a number of records have a cooling bias, especially in the tropics. While there remain disparities among different tropospheric temperature trends estimated from satellite Microwave Sounding Unit (MSU and advanced MSU) measurements since 1979, and all likely still contain residual errors, estimates have been substantially improved (and data set differences reduced) through adjustments for issues of changing satellites, orbit decay and drift in local crossing time (i.e. diurnal cycle effects). It appears that the satellite tropospheric temperature record is broadly consistent with surface temperature trends provided that the stratospheric influence on MSU channel 2 is accounted for. The range (due to different data sets) of global surface warming since 1979 is 0.016 °C to 0.018 °C per decade compared to 0.012 °C to 0.019 °C per decade for MSU estimates of tropospheric temperatures. It is likely, however, that there is slightly greater warming in the troposphere than at the surface, and a higher tropopause, with the latter due also to pronounced cooling in the stratosphere.'
Ocean temperatures and sea level
Most of the heating caused by the enhanced greenhouse effect is stored in the world's oceans. This warming is clearly seen in changes in sea surface temperature, ocean heat content and sea-level rise due to thermal expansion of water (Domingues et al., 2008). Figure 2 shows estimates of near-global ocean heat content for the upper 300 metres and 700 metres of the ocean between 1950 and 2003, along with sea surface temperatures. Warming of the oceans is clearly evident.
Figure 2: Comparison of near global ocean heat content for the upper 700 metres (thick blue line, light blue shading indicates an estimate of one standard deviation error) and upper 100 metres (thick orange line; thin orange lines indicate estimates of one standard deviation error). Sea surface temperatures (aqua; right-hand scale) are also shown. All time series are smoothed with a three-year running average and are relative to 1961. The global mean stratospheric optical depth (brown, arbitrary scale) at the bottom indicates the timing of major volcanic eruptions. The brown curve is a three-year running average of these values, included for comparison with the smoothed observations (Domingues et al. 2008).
Warming leads to melting of ice and thermal expansion of the oceans, both of which contribute to higher sea level. Global average sea-level has risen by 17 cm during the 20th century, and the rate of increase from 1961 to 2003 was 1.3 to 2.3 mm/yr (IPCC, 2007).
High quality measurements of near-global sea level have been made since late 1992 by satellite altimeters. These data have shown a fairly steady increase in global mean sea level of around 3.2 ± 0.4 mm/year from 1993 to present (Cazenave and Llovel, 2010; Church and White, 2011) (http://www.cmar.csiro.au/sealevel/sl_hist_last_15.html).
This is about 50 per cent larger than the average value over the 20th century. Whether this represents a further increase in the rate of sea level rise is not yet certain.
References
Cazenave A and Llovel W. 2010. Contemporary Sea Level Rise. Annual Review of Marine Science. 2: 145-173.
Church JA and White NJ. 2011. Changes in the rate of sea-level rise from the late 19th to the early 21st Century. Surveys in Geophysics. Submitted.
Domingues, C.M., Church, J.A., White, N.J., Gleckler, P.J., Wijffels, S.E., Barker, P.M. and Dunn, J.R. (2008). Improved estimates of upper-ocean warming and multi-decadal sea-level rise. Nature 453, doi:10.1038, pp. 1090–1094.
Easterling, D. R. and Wehner, M. F. (2009). Is the climate warming or cooling? Geophys. Res. Lett., 36, L08706, doi:10.1029/2009GL037810.
Fawcett, R. (2008). Has the world cooled since 1998? Bulletin of the Australian Meteorological and Oceanographic Society, 20, pp. 141–148.
IPCC (2007). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L.
Kaufmann R, Kauppi H, Mann M and Stock J. 2011. Reconciling anthropogenic climate change with observed temperature 1998–2008. PNAS.
Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. www.ipcc.ch.
Thorne et al. 2011. Tropospheric temperature trends: history of an ongoing controversy. WIREs Climate Change. DOI: 10.1002/wcc.80.
WMO (2011). 2010 equals record for world's warmest year. World Meteorological Organisation Press Release. No. 906. World Meteorological Organization. Geneva.