• GG 312: Global Climate Change and Environmental Impacts (Fall 2002)

     

    Chapter 3: Observed Climate Variability and Change

    3.1. INTRODUCTION
    3.2. HAS THE CLIMATE WARMED?
    3.2.A. Land Surface Temperature
    3.2.B. Sea Surface Temperature
    3.2.C. Land and Sea Surface Temperature Combined
    3.2.D. Spatial Distribution of Surface Temperature Trends
    3.2.E. Changes in Diurnal Temperature Range
    3.2.F. Trends in Tropospheric Temperature
    3.2.G. Changes in the Cryosphere
    3.2.H. Summary
    3.3. HAS THE CLIMATE BECOME WETTER?
    3.3.A. Land Precipitation
    3.3.B. Ocean Precipitation
    3.3.C. Water Vapor
    3.3.D. Evaporation (Land)
    3.3.E. Summary
    3.4. ARE THE ATMOSPHERE/OCEAN CIRCULATIONS CHANGING?
    3.4.A. ENSO
    3.4.B. NAO
    3.4.C. Summary
    3.5 HAS THE CLIMATE BECOME MORE EXTREME OR VARIABLE?
    3.6 IS THE 20th CENTURY WARMING UNUSUAL?
    3.7 ARE THE OBSERVED TRENDS INTERNALLY CONSISTENT?

    3.1. Introduction

    This chapter focuses on 6 questions -

    • Has the climate warmed?
    • Has the climate become wetter?
    • Are the atmosphere/ocean circulations changing?
    • Has the climate become more variable or extreme?
    • Is the 20th century warming unusual?
    • Are the observed trends internally consistent?
    The answers to these questions critically depend on the availability of accurate, complete and consistent series of observations. If conclusions regarding trends cannot always be drawn, it does not necessarily mean that the trends are absent!

    3.2. Has the Climate Warmed?

    3.2.A. Land Surface Temperature

    The annual global average surface air temperature anomalies (C) for land areas is shown in Figure 3.1. The top panel in Figure 3.1 shows the CRU annual averages, together with an approximately decadally smoothed curve, to highlight decadal and longer changes. This is compared with smoothed curves from the other three analyses in the bottom panel. The plotted quantities are annual global average surface air temperature anomalies (C) for land areas, 1861 to 1999, relative to 1961 to 1990. Values are the simple average of the two hemispheres. Two standard error uncertainties are plotted about the CRU curve and include uncertainties due to urbanisation.

  • The two main periods of warming in all three series are between about 1910-1945 and between 1976-1999.

  • The rate of warming in land surface air temperature from 1976-1999 was about twice as fast (but interannually more variable) as that for the period 1910-1945.

  • Both periods of warming are statistically significant, as is (easily) the warming since 1861 or 1901.

  • The period 1946-1975 had no significant change of temperature.

  • The equivalent linear changes in global average CRU land surface air temperature over 1861-1999 and 1901-1999 that takes into account annual sampling errors and uncertainties due to urbanisation are 0.63 +/- 0.21 C and 0.61 +/- 0.18 C, respectively.

  • Corresponding Northern and Southern Hemisphere changes for 1901-1999 are 0.71 +/-0.23 C and 0.52 +/- 0.13 C, respectively.

    3.2.B. Sea Surface Temperature (SST)

    Figure 3.2 shows annual values of global SST anomalies (relative to 1961-1990), using a recently improved UK Met Office (UKMO) analysis that does not fill regions of missing data, together with decadally smoothed values of SST from the same analysis.

  • Night marine air temperatures (NMAT) are also shown. These generally agree well after 1900, but NMAT data are warmer before that time with a slow cooling trend from 1860 not seem in the SSTs, though the minimum around 1910 is seen in both series.

  • The SST analysis from IPCC-1995 is also shown. The changes in SST since IPCC-1995 are generally fairly small, though the peak warmth in the early 1940s is more evident in the more recent analysis, supported by the NMAT analysis.

  • The trends in global average UKMO and NMAT time series for the period 1901-1999 are 0.5 C and 0.6 C. These SST trends are comparable to that observed in LST.

    3.2.C. Land And Sea Surface Temperatures Combined

    Figure 3.3 summarises the relative changes of UKMO SST, UKMO NMAT and CRU land surface air temperature. The greater warming of the land in recent years is clear, but otherwise all three curves have a generally similar shape except that modest cooling of NMAT in the late nineteenth century is not seen in the SST data.

  • The global trend from 1861 to 1999 can be cautiously interpreted as an equivalent linear warming of 0.6C over the 139-year period, with a 95% confidence level uncertainty of +/-0.15C.

  • From 1901 an equivalent warming of 0.57C has occurred, with an uncertainty of +/-0.18C.

    3.2.D. Spatial Distribution of Surface Temperature Trends

    Most of the warming of the 20th century occurred in two distinct periods separated by several decades of little overall globally averaged change. Figures 3.4 and 3.5 highlight the worldwide behavior of temperature change in the three periods. These linear trends have been calculated from a gridded combination of UKMO SST and CRU temperatures. The periods chosen are 1910-1945 (first warming period), 1946-1975 (period of little global temperature change), 1976-1999 (second warming period, where all four seasons are shown in Figure 3.5) and the 20th century, 1901-1999.

  • It can be seen that there is a high degree of local consistency between the SST and land air temperature across the land-ocean boundary, noting that the corrections to SST are independent of the land data.

  • The 1910-1945 warming was greatest in, but not limited to, the North Atlantic, Arctic and northern North America.

  • By contrast, the period 1946-1975 shows widespread cooling in the Northern Hemisphere relative to much of the Southern. Much of the cooling was seen in the Northern Hemisphere regions that showed most warming in 1910-1945.

  • In accord with the results in IPCC-1995, recent warming has been greatest over the mid latitude Northern Hemisphere continents in winter.

  • Over 1901-99 as a whole, warming is seen everywhere except south of Greenland and in a few scattered continental regions in the tropics or subtropics. Faster warming of the land surface temperature than the ocean surface temperature in the last two decades could in part be a signal of anthropogenic warming.

    We conclude that in the twentieth century we have seen a consistent large-scale warming of the land and oceansurface. Some regional details can be explained from accompanying atmospheric circulation changes.

    NAS: Surface Temperature Observations
    NASA GISS Analysis
    How is 2002 Shaping Up?

    3.2.E. Changes in Diurnal Temperature Range

    The increase in temperature in recent decades has involved a faster rise in daily minimum than daily maximum temperature in many continental regions. This gives a decrease in the diurnal temperature range (DTR) in many parts of the world. The trends in annual diurnal temperature range (DTR, C per decade), 1950-1993, for non-urban stations are shown in Figure 3.6 (reductions are in blue and increases in red).

  • The overall global trend for the maximum temperature during 1950-1993 is approximately 0.1 C/decade and the trend for the minimum temperature is about 0.2 C/decade. Consequently, the trend in the DTR is about -0.1 C/decade.

  • Since the DTR is the maximum temperature minus the minimum temperature, the DTR can decrease when the trend in the maximum or minimum temperature is downward, upward, or unchanging. This contributes to less spatial coherence on the DTR map.

  • The DTR decreased in most areas, except over middle Canada, and parts of southern Africa, south-west Asia, Europe, and the western tropical Pacific Islands.

    Max and Min Temperature Trends for the Globe

    3.2.F. Trends in Tropospheric Temperature

    The surface, tropospheric and stratospheric temperature variations since 1958 using representative data sets are shown in Figure 3.7. Especially consistent is the relative shift to warmer temperatures in the troposphere compared to the surface around 1977, followed by large variations due to ENSO (particularly in 1998) and volcanic events (El Chichon in 1982 and Mt. Pinatubo in 1991).

    Global variations and trends in the lower stratosphere are temporally more coherent than in the troposphere (Figure 3.8). The warming effects due to the volcanic eruptions are clearly evident. All stratospheric data sets indicate significant negative trends. Blended information for 5 km thick levels in the stratosphere at 45N show a negative trend temperature increasing with height: -0.5C/decade at 15 km, -0.8C/decade at 20-35 km, and -2.5C/decade at 50 km. These large, negative trends are consistent with models of the combined effects of ozone depletion and increased concentrations of infrared radiating gases, mainly water vapour and carbon dioxide.

    It is very likely that the surface has warmed relative to the troposphere, and the troposphere has warmed relative to the stratosphere since 1979. There is evidence that the troposphere warmed relative to the surface in the pre-satellite era (1958-1979), though confidence in this finding is lower.

    NAS: Trend Comparisons

    3.2.G. Changes in the Cryosphere

  • Snow cover: Satellite records indicate that Northern Hemisphere annual snow cover extent (SCE) has decreased by about 10% since 1966 largely due to decreases in spring and summer since the mid-1980s over both the Eurasian and American continents (Figure 3.9).

  • Sea-ice extent: During November 1978 through December 2000, the sea ice extent over the Northern Hemisphere showed a decrease of -2.8% +/- 0.3% per decade (Figure 3.10). Related to the decline in sea ice extent is a decrease in the length of the sea ice season and an increase in the length of the Arctic summer melting season.

  • Sea-ice thickness: Our knowledge of sea ice thickness in the Arctic comes largely from upward sonar profiling by US and British submarines since 1958 and 1971 respectively. Late summer, September to October, data from 1993, 1996 and 1997 from a US civilian submarine research programme were compared to data from six summer cruises from the period 1958-1976. There was a mean reduction in thickness of 42% the earlier period to the present.

  • Permafrost: About 25% percent of the land mass of the Northern Hemisphere is underlain by permafrost, including large regions of Canada, China, Russia and Alaska. Over half of the worlds permafrost is at temperatures a few degrees below 0C. Recent analyses indicate that permafrost in many regions of the earth is currently warming.

  • Lakes and river ice: A recent analysis has been made of trends in very long Northern Hemisphere lake and river ice records over the 150-year period 1846-1995. Ice break-up dates now occur on average about nine days earlier in the spring than at the beginning of the record, and autumn freeze-up occurs on average about ten days later.

  • Glaciers: The general picture is one of widespread retreat. In a few regions a considerable number of glaciers are currently advancing very likely due to increases in precipitation due to the positive phase of the North Atlantic Oscillation.

    Boreholes and Tree Rings

    3.2.H. Summary

  • Global surface temperatures have increased between 0.4 and 0.8C since the late 19th century, but most of this increase has occurred in two distinct periods, 1910-45 and since 1976.

  • The rate of temperature increase since 1976 has been almost 0.2C per decade.

  • New analyses of mean daily maximum and minimum temperatures continue to support a reduction in the diurnal temperature range with minimum temperatures increasing at about twice the rate of maximum temperatures.

  • Seasonally, the greatest warming has occurred during the Northern Hemisphere winter and spring, but the disparity of warming between summer and winter has decreased.

  • Largest rates of warming continue to be found in the middle and high latitude continental regions of the Northern Hemisphere.

  • Analyses of overall temperature trends in the low to mid troposphere and near the surface since 1958 are in good agreement, with a warming of about 0.1C per decade.

  • Since the beginning of the satellite record (1979), however, low to mid troposphere temperatures have warmed in both satellites and weather balloons at a global rate of only about 0.05 C/decade. This is about 0.15 C/decade less than the rate of temperature increase near the surface since 1979. About half of this difference in warming rate is very likely to be due to the combination of differences in spatial coverage and the real physical affects of volcanoes, ENSO. The remaining difference remains unexplained, but is likely to be real.

    3.3. Has the Climate Become Wetter?

    Increasing global surface temperatures are very likely to lead to changes in precipitation and atmospheric moisture because of changes in atmospheric circulation, a more active hydrologic cycle, and increases in the water holding capacity throughout the atmosphere. Atmospheric water vapour is also a climatically critical greenhouse gas, and an important chemical constituent in the troposphere and stratosphere.

    3.3.A. Land Precipitation

    Overall, global land precipitation has increased by about 2% since the beginning of the 20th Century. The increase is statistically significant but has neither been spatially nor temporally uniform. Figure 3.11 shows trends for 1900-99 for the four seasons. Precipitation trends are represented by the area of the circle with green representing increases and brown representing decreases. Average trends within six latitude bands are shown in the legend of each map. Trends found to be significant under both tests are indicated with an asterisk.

    Mid and High Latitudes: During the 20th Century, annual-zonally averaged precipitation increased between 9% and 16% for the zones 30N to 85N and by about 2 to 5% between 0S to 55S during this time (Figure 3.12). Figure 3.11 shows mostly increasing precipitation in the Northern Hemisphere mid and high latitudes, especially during the autumn and winter, but these increases vary both spatially and temporally.

  • Precipitation over the United States has increased between 5-10% since 1900 but this increase has been interrupted by multi-year anomalies like the drought years of the 1930s and early 1950s.

  • Precipitation in Canada has increased by an average of more than 10% over the 20th Century.

  • Over the last 50 years there has been a slight decrease in annual precipitation over China.

  • There have been marked increases in precipitation in the latter part of the 20th Century over northern Europe.

  • Precipitation has increased since 1891 by about 5% west of 90E, accompanied by increases in streamflow and a rise in the level of the Caspian Sea. Soil moisture data for large regions of Eurasia show large upward trends.

  • Annual total rainfall has increased over much of Australia with significant significant increases of 15-20% in large areas.

    Tropics and Sub-Tropics: The increase of precipitation in the middle and high latitudes contrasts with decreases in the northern subtropics.

  • There is little evidence for a long-term trend in Indian monsoonal rainfall but there are multi-decadal variations.

  • There has been a pattern of continued aridity since the late 1960s throughout North Africa south of the Sahara. The driest period was in the 1980s with some recovery occurring during the 1990s.

    Sahelian Drought
    Sahel Rainfall
    Rainfall Variability and Drought in Sub-Saharan Africa

    3.3.B. Ocean Precipitation

    The strong spatial variability inherent in precipitation requires the use of estimates based on satellite observations for many regions. Thus satellite data are essential to infer global changes of precipitation, as the oceans account for 70% of the global surface area. The first satellite instrument specifically designed to make estimates of precipitation did not begin operation until 1987, but this record it is too short to draw conclusions.

    3.3.C. Water Vapor

    In situ and radiosonde measurements tend to show increasing water vapour in the lower troposphere and near the surface, though this is not seen everywhere, and data quality is still an issue.

    3.3.D. Evaporation (Land)

    Evaporation increased during the second half of the 20th century over most dry regions of the United States and Russia. The increase is related to the greater availability of moisture at the surface due to increases in precipitation and the higher temperatures.

    3.3.E. Summary

  • Since IPCC-1995, land surface precipitation has continued to increase in the Northern Hemisphere mid and high latitudes; over the subtropics, the drying trend has been ameliorated somewhat.

  • Where data are available, changes in annual streamflow relate well to changes in total precipitation.

  • Little can be said about changes in ocean precipitation as satellite data sets have not yet been adequately tested for time-dependent biases.

  • Changes in water vapor have been analyzed most for selected Northern Hemisphere regions, and show an emerging pattern of surface and tropospheric water vapor increases over the past few decades.

  • Over land, an increase in cloud cover of a few percent since the turn of the century is observed, which is shown to closely relate to changes in the diurnal temperature range.

    Precipitation Trends in the 20th Century

    3.4. Are the Atmosphere/Ocean Circulations Changing?

    Changes or fluctuations in atmospheric and oceanic circulation are important elements of climate. Such circulation changes are the main cause of variations in climate elements on a regional scale. El Nino Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) are such examples.

    3.4.A. ENSO

    ENSO is the primary global mode of climate variability in the 2-7 year time band. El Nino is defined by SST anomalies in the eastern tropical Pacific while the Southern Oscillation (SO) is a measure of the atmospheric circulation response in the Pacific-Indian Ocean region.

    Multiproxy-based reconstructions of the behaviour of ENSO have recently been attempted for the past few centuries. Figure 3.13 compares the behaviour of two such reconstruction series with recent ENSO behaviour. The SOI reconstruction has been rescaled to have the sign and variance of the Nino3 reconstruction; the two reconstructions, based on independent methods and partially independent data, have a linear correlation r=0.64 during the pre-calibration interval. While the estimated uncertainties in these reconstructed series are substantial, they suggest that the very large 1982-83 and 1997-98 warm events might be outside the range of variability of the past few centuries.

    Instrumental records have been examined to search for possible changes in ENSO over the past 120 years. Three new reconstructions of SST in the eastern Equatorial Pacific (Figure 3.14) that use optimum interpolation methods exhibit strong similarities. The dominant 2-6 year timescale in ENSO is apparent. Both the activity and periodicity of ENSO have varied considerably since 1871 with considerable irregularity in time. There was an apparent shift in the temperature of the tropical Pacific around 1976 to warmer conditions, discussed in IPCC-1995, which appeared to continue until at least 1998. During this period ENSO events were more frequent, intense or persistent. It is unclear whether this warm state continues, with the long La Nina from late 1998 until mid 2000. Whether global warming is influencing El Nino, especially given the remarkable El Nino of 1997-1998, is a key question, especially as El Nino affects global temperature itself.

    ENSO: BACKGROUND

    A warm southward current appears once every 3 to 4 years off the west coast of Peru, moderating the low surface temperature during the early months of the year. This has come to be known as El Nino or the Christ Child. Our understanding of El Nino has increased substantially in the last 30 years, so much so, that models of this phenomenon can be used to predict these events.

    El Nino's result in a warm pool of waters over the entire tropical equatorial Pacific. The opposite event, La Nina, or the cold event, is manifest as a large pool in the tropical Pacific having cooler surface waters. These terms El Nino and La Nina refer to sea surface temperature oscillations in the equatorial Pacific.

    During an El Nino there is high pressure at Darwin, Australia, and low pressure area near Tahiti, in the mid-equatorial Pacific. Therefore, the Southern Oscillation Index (SOI) defined as the difference in surface level pressure between Tahiti and Darwin, is negative during an El Nino event. Likewise, during the La Nina events, the SOI is positive.

    The coupling between the ocean and the atmosphere in the climate system results in this phenomenon, which is collectively called the El Nino Souther Oscillation (ENSO). ENSO events are important because they are the most dominant source of interannual variability in all climatic fields. Variations in climate due to ENSO events can result in billions of dollars worth of damage -

    • Droughts in Australia, Northeastern Brazil, Southeastern South America, Midwestern US, etc.
    • Collapse of agriculture, fishing industry, bird populations, etc.
    • Forest fires in fareastern Asia, Amazonia, etc.

    A schematic view of the tropical Pacific atmospheric and oceanic conditions towards the end of a year in which La Nina is shown in the top panel of Figure 3.15. Exceptionally intense trade winds come together in the Intertropical Convergence Zone (ITCZ), which is relatively far North, and the huge convective zone to the west of the date line, where sea surface temperatures exceed 28C.The eastward surface current between 3N and 10N, which is known as the North Equatorial Countercurrent because it flows counter to the prevailing winds, is relatively weak. The westward South equatorial current is extremely strong, especially near the equator, where divergent motions cause intense upwelling and hence low sea surface temperatures. The thermocline, the layer of large vertical temperature gradients that separates the warm surface waters from the cold waters at depth, slopes steeply to the west, where its depth is approximately 150 meters.

    The conditions towards the end of a year in which El Nino occurs are just the opposite of those prevailing during La Nina, shown in the bottom panel of Figure 3.15. The trade winds have collapsed, to be replaced in the west by westerly winds. The eastward movement of the convective zone is associated with an eastward expansion of warm surface waters, a thermocline that is elevated in the west and depressed in the east, an intensified eastward North Equatorial countercurrent, a weakened westward South Equatorial Current that is replaced by an eastward equatorial jet in the west, and very weak equatorial upwelling.

    Learn about ENSO here
    About ENSO
    About ENSO (very neat!)
    About ENSO (very very cool!)
    Current State of ENSO
    Current State of ENSO (I mean really current!)

    3.4.B. NAO

    The atmospheric circulation over the Northern Hemisphere has exhibited anomalous behaviour over the past several decades. The dominant pattern of atmospheric circulation variability over the North Atlantic is known as the North Atlantic Oscillation (NAO), and its wintertime index is shown in Figure 3.16. As discussed in IPCC-1995, positive values of the NAO give stronger-than-average westerlies over the middle latitudes of the Atlantic with low SLP anomalies in the Icelandic region and over much of the Arctic and high SLP anomalies across the subtropical Atlantic and into southern Europe. The positive phase of the NAO is associated with cold winters over the northwest Atlantic and warm winters over Europe, Siberia and eastern Asia as well as wet conditions from Iceland to Scandinavia and dry winters over southern Europe.

    A sharp reversal is evident in the NAO index starting around 1970 from a negative towards a positive phase. Since about 1985, the NAO has tended to remain in a strong positive phase, though with substantial interannual variability. The recent upward trend in the NAO accounts for much of the regional surface winter half year warming over Northern Europe and Asia north of about 40N over the past 30 years, as well as the cooling over the northwest Atlantic. Moreover, when circulation changes over the North Pacific are also considered, much of the pattern of Northern Hemisphere winter half year surface temperature changes since the mid 1970s can be explained. This can be associated with changes in the NAO, and in the Pacific North American (PNA) atmospheric pattern related to ENSO.

    The NAO is regarded (largely) by some as the regional expression of a zonally symmetric hemispheric mode of variability characterised by a seesaw of atmospheric mass between the polar cap and the middle latitudes in both the Atlantic and Pacific Ocean basins. This mode has been named the Arctic Oscillation (AO). The time series of the NAO and AO are quite similar (Figure 3.16): the correlation of monthly anomalies of station data SLP series of NAO and AO is about 0.7. The recent strength of the positive phase of the NAO seems unusual from several reconstructions but these proxy data reconstructions may underestimate variability.

    NAO: Background

    3.4.C. Summary

  • The interannual variability of ENSO has varied substantially over the last century, with notably reduced variability during the period 1920-60, compared to adjacent periods.
  • It remains unclear whether global warming has influenced the shift towards less frequent La Nina episodes since 1976, including the abnormally protracted ENSO 1990-95 event and the exceptionally strong 1982-83 and1997-98 events.
  • In the Northern Hemisphere, pronounced changes in winter atmospheric and oceanic circulations over the North Pacific in the 1970s (the North Pacific Oscillation) have been paralleled by wintertime circulation changes over the North Atlantic, recorded by the NAO.

    3.5. Has the Climate Become More Extreme or Variable?

    Changes in climate variability and extremes of weather and climate events have received increased attention in the last few years. Understanding changes in climate variability and climate extremes is made difficult by interactions between the changes in the mean and variability. The distribution of temperatures often resembles a normal distribution. An increase in the mean leads to new record high temperatures (Figure 3.17a), but a change in the mean does not imply any change in variability. For example, in Figure 3.17a, the range between the hottest and coldest temperatures does not change. An increase in variability without a change in the mean implies an increase in the probability of both hot and cold extremes as well as the absolute value of the extremes (Figure 3.17b). Increases in both the mean and the variability are also possible (Figure 3.17c), which affects (in this example) the probability of hot and cold extremes, with more frequent hot events with more extreme high temperatures and fewer cold events.

  • It is likely that there has been a widespread increase in heavy and extreme precipitation events in regions where total precipitation has increased, e.g., the mid and high latitudes of the Northern Hemisphere.
  • Increases in the mean have often been found to be amplified in the highest precipitation rates total.
  • In some regions, increases in heavy rainfall have been identified where the total precipitation has decreased or remained constant, such as eastern Asia. This is attributed to a decrease in the frequency of precipitation.
  • Fewer areas have been identified where decreases in total annual precipitation have amplified as decreases in the highest precipitation rates.
  • Temperature variability has decreased on intra-seasonal and daily time scales in limited regional studies.
  • New record high night-time minimum temperatures are lengthening the freeze-free season in many mid and high latitude regions.
  • The increase of global temperatures has resulted mainly from a significant reduction in the frequency of much below-normal seasonal mean temperatures across much of the globe, with a corresponding but smaller increase in the frequency of much above normal temperatures.
  • There is little sign of long-term changes in tropical storm intensity and frequency.
  • Recent analyses of changes in severe local weather (tornadoes, thunder days, lightning and hail) in a few selected regions provide no compelling evidence for widespread systematic long-term changes.

    3.6. Is the 20th Century Warming Unusual?

    To determine whether 20th century warming is unusual, it is essential to place it in the context of longer-term climate variability. Owing to the sparseness of instrumental climate records prior to the 20th century (especially prior to the mid 19th century), estimates of global climate variability during past centuries must often rely upon indirect proxy indicators--natural or human documentary archives that record past climate variations, but must be calibrated against instrumental data for a meaningful climate interpretation.

    Coarsely resolved climate trends over several centuries are evident in many regions e.g., from the recession of glaciers or the geothermal information provided by borehole measurements. Large-scale estimates of decadal, annual or seasonal climate variations in past centuries, however, must rely upon sources that resolve annual or seasonal climatic variations. Such proxy information includes width and density measurements from tree rings, layer thicknesses from laminated sediment cores, isotopes from ice cores and corals, etc.

    IPCC-1995 examined evidence for climate change in the past, on timescales of centuries to millennia. Based on information from a variety of proxy climate indicators, reconstructions of mountain glacier mass and extent, and geothermal subsurface information from boreholes, it was concluded that summer temperatures in the Northern Hemisphere during recent decades are the warmest in at least six centuries. Since IPCC-1995, a number of studies based on considerably expanded data bases of paleoclimate information have allowed more decisive conclusions about the spatial and temporal patterns of climate change in past centuries.

    There have been several attempts to combine various types of high-resolution proxy climate indicators to create large-scale paleoclimate reconstructions. Mann et al reconstructed global patterns of annual surface temperature several centuries back in time. They calibrated a combined terrestrial (tree ring, ice core, and historical documentary indicator) and marine (coral) multiproxy climate network against dominant patterns of 20th century global surface temperature. Averaging the reconstructed temperature patterns over the far more data-rich Northern Hemisphere half of the global domain, they estimated Northern Hemisphere mean temperature back to AD 1000 (Figure 3.18). The uncertainties (the shaded region in Figure 3.18) expand considerably in earlier centuries because of the sparse network of proxy data. Taking this into account, Mann et al concluded that the 1990s were likely to have been the warmest decade, and 1998 the warmest year, of the past millennium for at least the Northern Hemisphere.

    The largely independent multiproxy Northern Hemisphere temperature reconstructions of Jones et al. and Mann et al. are compared in Figure 3.19, together with an independent (extratratropical, warm-season) Northern Hemisphere temperature estimate from tree-ring density data. The estimated uncertainties shown are those for the smoothed Mann et al series. Significant differences between the three reconstructions are evident during the 17th and early 19th centuries where either the tree-ring or the Jones et al series lie outside the estimated uncertainties in the Mann et al series. Much of these differences appear to result from the different latitudinal and seasonal emphases of the temperature estimates. This conclusion is supported by the observation that the Mann et al hemispheric temperature average, when restricted to just the extratropical (30-70N band) region of the Northern Hemisphere, shows greater similarity in its trend over the past few centuries to the Jones et al reconstruction. The differences between these reconstructions emphasize the importance of regional and seasonal variations in climate change.

    Summary: The warming of the 20th century has a convincing global signature and is consistent with the paleoclimate evidence that the rate and magnitude of global or hemispheric surface 20th Century warming is very likely to have been the largest of the millennium, with the 1990s and 1998 likely to have been the warmest decade and year, respectively, in the Northern Hemisphere.

    A Paleo Perspective on Global Warming

    3.7. Are the Observed Trends Internally Consistent?

    It is very important to compare trends in the various indicators to see if a physically consistent picture emerges as this will critically affect the final assessment of our confidence in any such changes. A number of qualitative consistencies among the various indicators of climate change have increased our confidence in our analyses of the historical climate record: Figure 3.20 and Figure 3.21 summarizes the changes in various temperature and hydrological indicators respectively, and provide a measure of confidence about each change. Of particular relevance are the changes identified below:

  • Temperature over the land and oceans, with two estimates for the latter, are measured and adjusted independently, yet all three show quite consistent increasing trends (0.51 to 0.61 C/Century) over the 20th Century.
  • The nearly worldwide decrease in mountain glacier extent and mass is consistent with 20th century global temperature increases. A few recent exceptions in maritime areas have been affected by atmospheric circulation variations and related precipitation increases.
  • Though less certain, substantial proxy evidence points to the exceptional warmth of the late 20th Century relative to the last 1000 years. The 1990s are likely to have been the warmest decade of the past 1000 years over the Northern Hemisphere as a whole.
  • Satellite and balloon measurements agree that lower tropospheric temperatures have increased only slightly since 1979, though there has been a faster rate of surface temperature increase.. Balloon measurements indicate a larger lower tropospheric temperature increase since 1958, similar to that shown by global surface temperature measurements. Balloon and satellite measurements agree that lower stratospheric temperatures have declined significantly since 1979.
  • Trends of world-wide land surface temperatures (as opposed to combined land and ocean temperatures) derived from weather stations are in close agreement with satellite derived temperatures of the low-to-mid troposphere. This suggests that urban heat island biases are not significantly affecting surface temperatures.
  • The decrease in the continental diurnal temperature range since around 1950 coincides with increases in cloud amount and, at least since the middle 1970s in the Northern Hemisphere, increases in water vapor.
  • Decreases in spring snow cover extent since the 1960s and in the duration of lake and river ice over at least the last century, relate well to increases in Northern Hemispheric surface air temperatures.
  • The systematic decrease of spring and summer Arctic sea-ice extent in recent decades is broadly consistent with increases of temperature over most of the adjacent land and ocean. The large reduction in the thickness of summer and early autumn Arctic sea ice over the last 30-40 years is consistent with this decrease in spatial extent, but we are unsure to what extent poor temporal sampling and multidecadal variability are affecting the conclusions.
  • The increases in lower tropospheric water vapor and temperature since the mid 1970s are qualitatively consistent with an enhanced hydrologic cycle. This is in turn consistent with a greater fraction of precipitation being delivered from extreme and heavy precipitation events, primarily in areas with increasing precipitation, e.g., middle and high latitudes of the Northern Hemisphere.
  • Where data are available, changes in precipitation generally correspond with consistent changes in streamflow and soil moisture.

    Summary: We conclude that the variations and trends of the examined indicators consistently and very strongly support an increasing global surface temperature over at least the last century, though substantial shorter term global and regional deviations from this warming trend are very likely to have occurred.

    Whither U.S. Climate?

    Oct-16-2002
    rmyneni@bu.edu