188.8.131.52 Synthesis of studies
Next, a synthesis of the significant observed changes described in Section 1.3 and the observed regional temperatures over the last three decades was performed. Significant observed changes documented since the TAR were divided into the categories of cryosphere, hydrology, coastal processes, marine and freshwater biological systems, terrestrial biological systems, and agriculture and forestry, as assessed in Section 1.3. Studies were selected that demonstrate a statistically significant trend in change in systems related to temperature or other climate change variable as described by the authors, for the period 1970 to 2004 (study periods may be extended later), with at least 20 years of data. Observations in the studies are characterised as ‘change consistent with warming’ and ‘change not consistent with warming’.
Figure 1.8 shows the warming trends over the period 1970 to 2004 (from the GHCN-ERSST dataset; Smith and Reynolds, 2005) and the geographical locations of significant observed changes. A statistical comparison shows that the agreement between the regions of significant and regional warming across the globe and the locations of significant observed changes in systems consistent with warming is very unlikely to be due to natural variability in temperatures or natural variability in the systems (Table 1.12) (see also Supplementary Material).
For regions where there are both significant warming and observed changes in systems, there is a much greater probability of finding coincident significant warming and observed responses in the expected direction. The statistical agreement between the patterns of observed significant changes in systems and the patterns of observed significant warming across the globe very likely cannot be explained by natural climate variability.
Figure 1.8. Locations of significant changes in observations of physical systems (snow, ice and frozen ground; hydrology; coastal processes) and biological systems (terrestrial, marine and freshwater biological systems), are shown together with surface air temperature changes over the period 1970 to 2004 (from the GHCN-ERSST datatset). The data series met the following criteria: (1) ending in 1990 or later; (2) spanning a period of at least 20 years; (3) showing a significant change in either direction, as assessed by individual studies. White areas do not contain sufficient observational climate data to estimate a temperature trend.
Uncertainties in observed change studies at the regional level relate to potential mismatches between climate and system data in temporal and spatial scales and lack of time-series of sufficient length to determine whether the changes are outside normal ranges of variability. The issue of non-climate driving forces is also important. Land-use change, changes in human management practices, pollution and demography shifts are all, along with climate, drivers of environmental change. More explicit consideration of these factors in observed change studies will strengthen the robustness of the conclusions. However, these factors are very unlikely to explain the coherent responses that have been found across the diverse range of systems and across the broad geographical regions considered (Figure 1.9).
Since systems respond to an integrated climate signal, precise assignment of the proportions of natural and anthropogenic forcings in their responses in a specific grid cell is difficult. The observed continent-averaged warming in all continents except Antarctica over the last 50 years has been attributed to anthropogenic causes (IPCC, 2007, Summary for Policy Makers). The prevalence of observed changes in physical and biological systems in expected directions consistent with anthropogenic warming on every continent and in some oceans means that anthropogenic climate change is having a discernible effect on physical and biological systems at the global scale.
Table 1.12. Global comparison of significant observed changes in physical and biological systems with regional temperature changes. Fraction of 5°*5° cells with significant observed changes in systems (from studies considered in this chapter) and temperature changes (for 1970 to 2004 from the GHCN-ERSST dataset (Smith and Reynolds, 2005)) in different categories (significant warming, warming, cooling, significant cooling). Expected values shown in parentheses are for the null hypotheses:
|Temperature cells ||Cells with significant observed change consistent with warming ||Cells with significant observed change not consistent with warming |
|Significant warming ||50% (2.5%) ||7% (2.5%) |
|Warming ||34% (22.5%) ||6% (22.5%) |
|Cooling ||3% (22.5%) ||0% (22.5%) |
|Significant cooling ||0% (2.5%) ||0% (2.5%) |
|Chi-squared value (significance level) || ||369 (<<1%) |
Figure 1.9. Changes in physical and biological systems and surface temperature. Background shading and the key at the bottom right show changes in gridded surface temperatures over the period 1970 to 2004 (from the GHCN-ERSST dataset). The 2*2 boxes show the total number of data series with significant changes (top row) and the percentage of those consistent with warming (bottom row) for (i) continental regions; North America, Latin America, Europe, Africa, Asia, Australia and New Zealand, and Polar Regions; and (ii) global-scale: Terrestrial (TER), Marine and Freshwater (MFW), and Global (GLO). The numbers of studies from the seven regional boxes do not add up to the global totals because numbers from regions except Polar do not include the numbers related to Marine and Freshwater systems. White areas do not contain sufficient observational climate data to estimate a temperature trend.