Specifying ACIA future time slices and climatological baseline

Transcription

1 Specifying ACIA future time slices and climatological baseline Vladimir Kattsov and Stanislav Vavulin Voeikov Main Geophysical Observatory, St.Petersburg, Russia 1. ACIA future time slices Specific time slices are useful to define the physical environment and possible ecosystem impacts in the future. Three specific twenty-year time slices ( , , and ) have been agreed on for the ACIA assessment and are centered on the years 2020, 2050, and 2080 (Källén et al., 2001). These times were chosen to give representative near-term, mid-term and longer-term outlooks for future changes, as well as to provide consistency with IPCC. The 20-year-slice approach is also adopted in the Coupled Model Intercomparison Project (CMIP, see Meehl et al., 2000), initially considered by ACIA as a source of AOGCM projections (Räisänen, 2001; Weatherhead et al., 2002). 2. ACIA climatological baseline Climatological baseline is a period of years representing the current climate, the latter being understood as a statistical description in terms of the mean and variability over the period. To satisfy widely adopted IPCC (1994) criteria, a baseline period should: - be representative of the present-day or recent average climate in the region considered; - be of sufficient duration to encompass a range of climatic variations; - cover a period for which data on all major climatological variables are abundant, adequately distributed over space and readily available; - include data of sufficiently high quality for use in evaluating impacts; - be consistent or readily comparable with baseline climatologies used in other impact assessments. Until now, the most widely used baseline period has been the WMO-defined classical 30-year period. Usually it was (particularly, in all three IPCC Assessment Reports). In some cases, an earlier period was also used. It is expected that the IPCC Fourth Assessment Report will use the new climatological baseline (IPCC-TGCIA, 1999), which was not available for the three IPCC assessments. The 20-year period has been selected as the ACIA climatological baseline. While being shorter than the WMO standard, the ACIA baseline coincides in its duration with the ACIA future time slices. Another (technical) reason for selecting the baseline period, rather than, e.g., the , was the availability of the former (but not the latter) in the outputs of all five B2 simulations stored in the ACIA archive.

2 A possibility exists that the baseline duration of 20 years is insufficient to reflect natural climatic variability on a multidecadal timescale. However, such possibility is not excluded for the standard 30-year period as well (IPCC-TGCIA, 1999). Another point of concern is the fact that the ACIA climatological baseline includes at least 10 globally warmest years in the last about century and a half record (IPCC-TGCIA, 1999). Following the recommendations of IPCC-TGCIA, a comparison of the alternative (ACIA) climatological baseline ( ) against the standard period is given below for the first order climatic characteristics (surface air temperature, precipitation and mean sea level pressure). The most natural approach is to use for this purpose reanalysis data, e.g. NCEP/NCAR reanalysis (Kistler et al., 2001), and an ACIA AOGCM output covering the both baseline periods (e.g. HadCM3). 3. Intercomparison of the ACIA and the standard WMO climatological baselines as obtained from the NCEP/NCAR reanalysis Tables 1 and 2 give seasonal and annual multiyear/area averages of the primary atmospheric variables for the two baseline periods obtained from the NCEP/NCAR reanalysis. The global means are presented in Table 1, and the polar ( N) means in the Table 2. Table 1. NCEP/NCAR-reanalysis-derived multiyear global means of the surface air temperature (SAT), precipitation (P), and mean sea level pressure (SLP) averaged over the WMO standard (61-90) and ACIA (81-00) climatological baselines. Global DJF MAM JJA SON annual baseline SAT, 0 C P, mm/d SLP, hpa Table 2. Same as Table 1, but for the polar area N N DJF MAM JJA SON annual baseline SAT, 0 C P, mm/d SLP, hpa The differences between the two baselines in the global means are systematic, but small. Globally, the ACIA baseline period is warmer by C in all seasons. The differences in global precipitation and SLP are negligible. As to the polar region within 60 0 N latitude, the differences between the two baselines are larger. The difference in the surface air temperature achieves its maximum in winter (0.7 0 C), and is the smallest in summer (0.2 0 C). ACIA baseline annual mean

3 precipitation is the same as the mean, and the SLP is slightly lower (by 1-2 hpa, if at all). Geographically, the differences between the two baselines in the NCEP/NCAR reanalysis are more pronounced (Figures 1 and 2). The Arctic region is generally warmer during the ACIA baseline period, especially in autumn. The strongest warming is evidently associated with the marginal sea-ice zone, particularly along the east coast of Greenland. For the ACIA baseline period, SLP is generally lower (by up to about 1.5 hpa) over the central Arctic and northern North Atlantic. Figure 1. Differences in the surface air temperature ( 0 C) seasonal and annual multiyear means in the northern polar cap ( N) between the ACIA ( ) and the standard WMO ( ) climatological baselines, obtained from NCEP/NCAR reanalysis.

4 Figure 2. Same as Figure 1, but for SLP (hpa).

5 4. Intercomparison of the ACIA and the standard WMO climatological baselines as simulated by HadCM3 Tables 3 and 4 give seasonal and annual multiyear/area averages of the atmospheric variables for the two baseline periods obtained from the HadCM3 B2 simulation. The global means are presented in Table 3, and the polar ( N) means in the Table 4. The differences in the averaged primary climatic variables between the two baseline periods simulated by the HadCM3 are quite comparable to those obtained from the NCEP/NCAR reanalysis. In the HadCM3 simulation, the ACIA baseline period is globally warmer by C, and wetter by mm/day, than the standard one. The differences in SLP are negligible. The differences in the polar region within 60 0 N latitude are qualitatively the same, but more pronounced. The warming ranges from C in summer to C in autumn, with the annual mean value of C. Precipitation increases by up to 0.04 mm/day (in autumn), with the annual mean increase of 0.03 mm/day. SLP tends to slightly decrease. Table 3. Same as Table 1, but for HadCM3 B2 simulation. Global DJF MAM JJA SON annual baseline SAT, 0 C P, mm/d SLP, hpa Table 4. Same as Table 3, but for the polar area N N DJF MAM JJA SON annual baseline SAT, 0 C P, mm/d SLP, hpa Geographically, in the HadCM3 simulation, the ACIA baseline period is warmer than the older standard throughout the year over the greater part of the Arctic. However, a cooling is simulated in winter over the western (mostly terrestrial) Arctic, and in all seasons over the Norwegian and Barents seas. The patterns of SLP baseline differences over the Arctic region are less comparable between the HadCM3 simulation and the NCEP/NCAR reanalysis. A key finding for all the variables is that the differences between the observational means for the Arctic during the two baseline periods are far smaller (except perhaps in summer) than the differences between the HadCM3-simulated means and the observed means for either period (compare Table 2 with Table 4)

6 Figure 3. Same as Figure 1, but for HadCM3. Figure 4. Same as Figure 2, but for HadCM3.

7 5. Intercomparison of the ACIA baseline period ( ) and the new standard ( ) as obtained from the NCEP/NCAR reanalysis. Having in mind that the is expected to be superseded by as a new standard 30-year averaging period, it is worthwhile to compare the ACIA climatological baseline against the latter. Figure 6 shows geographic distributions of seasonal and annual differences between the and the in surface air temperature. Figure 5. Differences in the surface air temperature ( 0 C) seasonal and annual multiyear means in the northern polar cap ( N) between the ACIA ( ) and the new standard ( ) climatological baselines, obtained from NCEP/NCAR reanalysis. The differences in the surface air temperature do not exceed C over the greater part of the Arctic (with an exception for autumn). The differences in precipitation and SLP (not shown here) are minor as well. 6. Summary The ACIA climatological baseline period ( ) satisfies the IPCC (1994) selection criteria. It has systematic but generally small differences with the widely adopted WMO standard baseline ( ) in the primary climatic variables. The differences can be easily taken into account when a comparison is needed between climate change scenarios employing the different baselines. More importantly, the differences

8 between Arctic means of the two baseline periods are much smaller than the differences between the corresponding HadCM3-simulated and the observed means for either period. An advantage of the ACIA climatological baseline period is that it is more current, than the period. The duration of the ACIA baseline period is exactly the same as that adopted for the ACIA future time slices. There are only minor differences between the ACIA baseline and the new standard baseline ( ), which is expected to supersede the one in the next IPCC assessments. The relative shortness of the ACIA baseline period (compared to the standard 30-year period) does not appear to be crucial from the perspective of the ACIA goals (ACIA, 2000). REFERENCES ACIA, 2000: Arctic Climate Impact Assessment (ACIA): An Assessment of Consequences of Climate Variability and Change and the Effects of Increased UV in the Arctic Region. Implementation Plan. Version 3.7 Prepared by the Assessment Steering Committee, 37 pp. IPCC, 1994: IPCC Technical Guidelines for Assessing Climate Change Impacts and Adaptations. Prepared by Working Group II [Carter, T.R., M.L. Parry, H. Harasawa, and S. Nishioka (eds.)] and WMO/UNEP, CGER-IO University College, London, UK and Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan, 59 pp. IPCC-TGCIA, 1999: Guidelines on the Use of Scenario Data for Climate Impact and Adaptation Assessment. Version 1. Prepared by Carter, T.R., M.Hulme, and M. Lal, Intergovernmental Panel on Climate Change, Task Group on Scenarios for Climate Impact Assessment, 69 pp. Källén, E., V. Kattsov, J. Walsh, and E. Weatherhead, 2001: Report from the Arctic Climate Impact Assessment Modeling and Scenarios Workshop. Stockholm, Sweden, January 29-31, pp. Kistler, R., E. Kalnay, W. Collins, S. Saha, G: White, J. Woollen, M. Chelliah, W. Ebisuzaki, M. Kanamitsu, V. Kousky, H. van den Dool, R. Jenne and M. Fiorino, 2001: The NCEP-NCAR 50-year reanalysis: Monthly means CD-ROM and documentation. Bull. Amer. Meteor. Soc, 82, Meehl, G. A., G. J. Boer, C. Covey, M. Latif, and R. J. Stouffer, 2000: The Coupled Model Intercomparison Project (CMIP). Bull. Am. Meteorol. Soc., 81, Räisänen, J., 2001: Intercomparison of 19 global climate change simulations from an Arctic perspective. In: Report from the Arctic Climate Impact Assessment Modeling and Scenarios Workshop. Stockholm, Sweden, January 29-31, 2001 [Källén, E., V. Kattsov, J. Walsh, and E. Weatherhead (eds.)], Smith, J.B., M. Hulme, J. Jaagus, S. Keevallik, A. Mekonnen, K. Hailemariam, 1998: Climate Change Scenarios. In: Handbook on Methods fro Climate Change Impact Assessment and Adaptation Strategies [J.F. Feenstra, I. Burton, J. Smith, and R.S.J. Tol (eds.)]. Version 2.0. United Nations Environment Programme and Institute for

9 Environmental Studies, Vrije Universiteit, Amsterdam, Weatherhead, E., J. Räisänen, J. Walsh, E. Källén, and V. Kattsov, 2002: Assessing Climate Change in the Arctic: Strategies for an Arctic Climate Impact Assessment (ACIA) (in preparation).