The Anthropocene The Instrumental Climate Record

Over the last two centuries, human activities have profoundly altered many aspects of the Earth's system, including its climate and its biogeochemistry, and to the point where some argue that we are entering a new geological era, the Anthropocene (Crutzen and Stoermer 2000; Steffen et al. 2007). In other words, the atmospheric concentration of some greenhouse gases has been enhanced to such an extent (IPCC 2007a) during the course of the Anthropocene, that regional to global changes in surface temperature and other climate parameters are to be expected.

The instrumental era of meteorological observations began with the invention of the thermometer by Galileo in the late sixteenth century and the mercury barometer by Torricelli in the seventeenth century. Instrumental records are, by far, the most reliable of all climate data, as they are precisely dated and employ physical calibrations. The modern instrumental measurements of climate parameters include thermometer-based surface temperatures from land regions and from the oceans, sea level pressure, continental and oceanic precipitation, sea ice extent, wind, and humidity. Unfortunately, it has only been since the late 1800s that enough stations have been systemically recording these data to permit reasonable estimates of global means.

There is a huge body of literature available addressing many aspects of climate change over the instrumental observation period. The most comprehensive assessment is published in the fourth assessment report of the IPCC (IPCC 2007a, Chap. 3). Only a few key points can be highlighted by this review.

4.1 Temperature Changes

Reliable global means of surface temperature are available since 1850. Several compilations by different research groups provide gridded monthly mean temperature estimates back through the mid nineteenth century on a large scale (Jones and Mann 2004). The reconstructions by the different groups are very similar, although there are some disagreements mainly in the early 1900s when station coverage was still sparse.

Figure 8 shows a time series of estimated annual global mean surface temperatures for the time period from 1850 to 2007 based on thermometer measurements; the values displayed in the figure are deviations from the mean of the base period 1961-1990. This time series is being compiled jointly by the Climatic Research

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Fig. 8 Global annual mean temperature anomalies (from the mean of the base period 1961-1990) for the years 1850-2007, the smoother line shows an 11-year running average. Data provided by Climatic Research Unit, Norwich, UK, and Hadley Centre, UK Met Office

1860 1880 1900 1920 1940 1960 1980 2000

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Fig. 8 Global annual mean temperature anomalies (from the mean of the base period 1961-1990) for the years 1850-2007, the smoother line shows an 11-year running average. Data provided by Climatic Research Unit, Norwich, UK, and Hadley Centre, UK Met Office

Unit and the UK Met. Office Hadley Centre; see Brohan et al. (2006) for details and error estimates. There is little temperature trend between 1850 and 1920, but between 1920 and 2007 the global mean temperature has risen by about 0.8°C. This has not been a steady rise though, temperature rose quickly during the 1920s to early 1940s, stabilized or fell slightly from the 1940s through to the late 1970s, and has again risen since 1980 to its present value. The 1990s were the warmest complete decade in the series. The warmest year of the entire series was 1998, with a temperature of 0.546 K above the mean of the current climatic period (1961-1990). Twelve of the thirteen warmest years in the series have now occurred in the past 13 years (1995-2007). The 10 warmest years on record are 1998, 2005, 2003, 2002, 2004, 2006, 2001, 2007, 2001, and 1997. Increased concentrations of greenhouse gases in the atmosphere (see e.g. Fig. 4 for CO2) due to human activities are most likely the underlying cause of warming in the twentieth century (IPCC 2007a).

The warming of the Earth is not distributed evenly over the globe, there are quite a few regional and seasonal differences to report. Continents show more rapid temperature increases compared to the oceans. A distinct warming can be observed in winter and spring in higher Northern latitudes, i.e. in the Arctic. Average Arctic temperatures increased at almost twice the global average rate over the past 100 years.

Widespread changes in extreme temperatures have also been observed over the last 50 years. Cold days, cold nights and frost have become less frequent while hot days, hot nights, and heat waves have become more frequent (IPCC 2007a; Easterling et al. 2000a). Several studies have shown that anthropogenic warming trends in Europe imply an increased probability of very hot summers. For example, the summer of 2003 with its record-breaking central European summer temperatures was probably the hottest since 1,500 (Stott et al. 2004), and a shift towards a regime with an increased temperature variability in addition to increases in mean temperature may have been responsible for this remarkable heat wave (Schar et al. 2004).

In addition to the changes in surface temperatures with a general warming tendency, analyses of balloon-borne and satellite measurements show a cooling of the stratosphere, which is consistent within greenhouse gas-driven climatic change. The observed pattern of tropospheric warming and stratospheric cooling is very likely due to the combined influences of greenhouse gas increases and stratospheric ozone depletion.

4.2 Other Changes 4.2.1 Hydrological Parameters

Due to a high variability of the water cycle on all time scales, significant trends in hydrological quantities are difficult to substantiate. Nevertheless, Huntington (2006) concludes a review of available studies that although data are often incomplete in spatial and temporal sense, the weight of evidence indicates an ongoing intensification of the water cycle. One of the central questions is, however, whether global warming leads to more evaporation and hence to an increased water vapour content in the atmosphere. Trenberth et al. (2005) observed increases in precipitable water over the global oceans since the mid-1980s. But caution is advised by Trenberth et al. (2005) for extracting information on trends in tropospheric water vapour from global reanalysis products, since they suffer from spurious variability and trends related to changing data quality and coverage. Satellite data presented by Soden et al. (2005) supports column-integrated moistening trends for the years from the mid-1980s. A most recently published evaluation of in situ surface air and dew point temperature data found very significantly increasing trends in global and Northern hemispheric specific humidity (Dai 2006). More details on evaporation changes are given in Quante and Matthias (2006).

The increased atmospheric moisture content associated with a warming might be expected to lead to increased global mean precipitation. However, precipitation is also strongly influenced by changes in the tropospheric energy budget and the atmospheric circulation, so spatio-temporal patterns in precipitation changes are likely to be complex. Concerning precipitation, the most recent IPCC reports states (IPCC 2007a): Global terrestrial annual mean precipitation showed a small upward trend over the twentieth century of approximately 2.1 mm per decade (based on the Global Historic Climatology Network, GHCN, data). Long-term trends from 1900 to 2005 have been observed in the amount of precipitation over many large regions. Significantly increased precipitation has been observed in Eastern parts of North and South America, Northern Europe and Northern and central Asia. The frequency of heavy precipitation events has increased since 1950 over most land areas, consistent with warming and observed increases of atmospheric water vapour. Drying has been observed in the Sahel, the Mediterranean, Southern Africa and parts of Southern Asia. Precipitation is highly variable spatially and temporally, and data are limited in some regions. More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Increased drying linked with higher temperatures and decreased precipitation have contributed to changes in drought (IPCC 2007a).

4.2.2 Cryogenic Components

The warming in the Arctic caused a decrease in snow cover since 1980, i.e. in spring time, as well as a decrease in sea ice of about 2.7% per decade, the latter is especially pronounced in late summer (September) showing a trend of 7.4% per decade. A record minimum has been observed in September 2007, which is not included in the reported trends. Also, in other parts of the globe changes in cryo-spheric components can be observed. Mountain glaciers and snow cover have declined on average in both hemispheres. Losses from the ice sheets of Greenland and Antarctica have very likely contributed to the observed sea level rise between 1993 and 2003 (IPCC 2007a). Flow speed has increased for some Greenland and

Antarctic outlet glaciers, which drain ice from the interior of the ice sheets. The corresponding increased ice sheet mass loss has often followed thinning, reduction or loss of ice shelves or loss of floating glacier tongues.

4.2.3 Sea Level Rise

Global average sea level rose at an average rate of 1.8 mm year-1 (range 1.3-2.3) over the years 1961-2003. The rate was faster over the years 1993-2003, with about 3.1 mm year-1 (range 2.4-3.8). Whether the faster rate for 1993-2003 reflects decadal variability or an increase in the longer-term trend is unclear. There is high confidence that the rate of observed sea level rise increased from the nineteenth to the twentieth century. The total twentieth century rise is estimated to be 0.17 m (range 0.12-0.22) (IPCC 2007a). An extended evaluation by Church, and White (2006) concluded a global mean sea-level rise from January 1870 to December 2004 of 0.195 m, a twentieth century rate of sea-level rise of 1.7 ± 0.3 mm year-1 and a significant acceleration of sea-level rise of 0.013 ± 0.006 mm year-2.

4.2.4 Wind

Mid-latitude westerly winds have strengthened in both hemispheres since the 1960s. There is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures (IPCC 2007a). There are also suggestions of increased intense tropical cyclone activity in some other regions (Emanuel 2005; Webster et al. 2005) where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones.

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