IGOR POLYAKOV
Igor V. Polyakov, Genrikh V. Alekseev, Roman V. Bekryaev, Uma Bhatt, Roger L. Colony, Mark A. Johnson, Valerii P. Karklin, Alexander P. Makshtas, David Walsh, and Alexander V. Yulin
[for more details read Polyakov et al., 2002 available on web http://www.frontier.iarc.uaf.edu/~igor/research/amplif/amplif_jul02_2.pdf ]
Positive feedbacks are believed to lead to enhanced high-latitude warming, as shown by analysis of observed surface air temperature (SAT) [Vinnikov et al., 1980] and as predicted by general circulation models (GCMs) [Manabe and Stouffer, 1994]. For example, in the ice-albedo feedback mechanism, warming leads to a reduction of ice and snow coverage, decreasing albedo, and resulting in further snow and sea ice retreat. Based on the polar amplification concept, one would consider high latitudes to be the area where global warming should be most pronounced. However, identification of enhanced high-latitude warming is complicated by strong natural variability, dominated by multidecadal fluctuations with a timescale of 60-80 years. This low-frequency variability in various climatically important parameters (dubbed ``LFO'' or ``low-frequency oscillation'' [Polyakov and Johnson, 2000]) is evident in many instrumental records from the Northern Hemisphere and the Arctic. The LFO may obscure possible long-term tendencies in the arctic climate system. In this study, we assess the concept of polar amplification of global warming via analysis of trends in the SAT records.
A composite time series of air-temperature anomalies for the Arctic northward of 62N is shown in Figure 1. This data is described in Polyakov et al. [2003a,b] and the data are available at http://www.frontier.iarc.uaf.edu/~igor/research/data/airtemppres.php). The LFO dominates SAT fluctuations, with two distinct warming periods (in the 1920-50s and from the mid-1970s to the present) and two cooling periods (prior to the 1920s and in the 1960-70s). Arctic warming in the 1930-40s was exceptionally strong, reaching 1.7C, compared with the 2000 maximum of 1.5C. The multidecadal LFO is stronger in the Arctic than in the Northern Hemisphere which may be attributed to the proximity of the Arctic to the North Atlantic, believed to be the origin of the LFO [e.g. Delworth and Mann, 2000].
Strong multidecadal variability influences the sign of air temperature trends (Figure 2, green). For example, during the previous 60 years (since the 1940s) arctic SAT trends are positive and are very large in the 1990s. However, arctic temperatures in the 1930-40s were exceptionally high, so trends calculated from the 1920s forward the data show a small but statistically significant cooling tendency. Extending the time series further back into the nineteenth century, the temperature trend again changes sign, signifying a general warming tendency over the entire record. Moreover, over the 125-year record we can identify periods when arctic trends were actually smaller or of different sign than Northern Hemisphere trends calculated using Jones et al. [1999] SAT data (Figure 2, red). This analysis underscores the inherent difficulty in differentiating between trends and long-term fluctuations. Computed arctic SAT trends depend on the phases and intensity of the LFO in addition to any underlying trend, whereas Northern Hemisphere trends do not show such a strong dependence on the LFO. Because the LFO is stronger in the Arctic, it is reasonable to expect a stronger LFO-driven modulation of trends in higher latitudes.
Perhaps the lack of polar amplification of global warming in SAT records is due to the moderating role of arctic ice? The ice-extent and fast ice thickness time series display a combination of decadal and multidecadal variability, with lower values prior to the 1920s, in the late 1930s-40s, and in recent decades, and higher values in the 1920s - early 1930s, and in the 1960-70s (Figure 3). This is consistent with the multi-year variability (LFO) evident in SAT records (Figure 1). Analysis of trends in these records shows that they are not statistically significant. Trends for recent decades seem to be larger but because of the fewer degrees of freedom in these shorter time records they are not statistically significant either. This is inconsistent with a hypothesis that sea ice has moderated arctic air temperatures in the last century, reducing atmospheric warming through ice melt.
Arctic variability is dominated by multi-decadal fluctuations. Incomplete sampling of these fluctuations results in highly variable arctic surface-air temperature trends. Modulated by multi-decadal variability, SAT trends are often amplified relative to northern-hemispheric trends, but over the 125-year record we identify periods when arctic SAT trends were smaller or of opposite sign than northern-hemispheric trends. Arctic and northern-hemispheric air-temperature trends during the 20th century (when multi-decadal variablity had little net effect on computed trends) are similar, and do not support the predicted polar amplification of global warming. The possible moderating role of sea ice cannot be conclusively identified with existing data. Observed long-term trends in arctic air temperature and ice cover are actually smaller than expected, and may be indicative of complex positive and negative feedbacks in the arctic climate system. In summary, if we accept that long-term SAT trends are a reasonable measure of climate change, then we conclude that the data do not support the hypothesized polar amplification of global warming.
Delworth, T. L., and M. E. Mann, 2000: Observed and simulated multidecadal variability in the Northern Hemisphere, Climate Dynamics, 16, 661-676.
Jones, P. D., M. New, D. E. Parker, S. Martin, and I. G. Rigor, 1999: Surface air temperature and its changes over the past 150 years, Review of Geophysics, 37(2), 173-199.
Manabe, S., and R. J. Stouffer, 1994: Multiple-century response of a coupled ocean-atmosphere model to an increase of atmospheric carbon dioxide, J. Climate, 7, 5-23.
Polyakov, I., and M. A. Johnson, 2000: Arctic decadal and inter-decadal variability, Geophys. Res. Lett., 27, 4097-4100. [download pdf]
Polyakov, I. V., R. V. Bekryaev, G. V. Alekseev, U. Bhatt, R. L. Colony, M. A. Johnson, A. P. Makshtas, 2003a: Variability and trends of air temperature and pressure in the maritime Arctic, 1875-2000, J. Climate,16(12), 2067-2077. (see http://www.frontier.iarc.uaf.edu/~igor/research/warm/warm_apr02.pdf).
Polyakov, I. V., G. V. Alekseev, R. V. Bekryaev, U. Bhatt, R. L. Colony, M. A. Johnson, V. P. Karklin, D. Walsh, and A. V. Yulin, 2003b: Long-term variability of ice in the arctic marginal seas, J. Climate,16(12), 2078-2085 (see http://www.frontier.iarc.uaf.edu/~igor/research/pdf/ice.pdf).
Polyakov, I., G. V. Alekseev, R. V. Bekryaev, U. Bhatt, R. Colony, M. A. Johnson, V. P. Karklin, A. P. Makshtas, D. Walsh, and A. V. Yulin, 2002: Observationally based assessment of polar amplification of global warming, Geophys. Res. Lett., 29,1878, doi:1029/2001GL011111. (see http://www.frontier.iarc.uaf.edu/~igor/research/amplif/amplif_jul02_2.pdf ).
Vinnikov K. Ya., G. V. Gruza, V. F. Zakharov, A. A. Kirillov, N. P. Kovyneva, and E. Ya. Ran'kova, 1980: Recent climatic changes in the Northern Hemisphere, Soviet Meteorology and Hydrology, 6, 1-10.
Composite time series of surface air temperature anomalies (degC) relative to 1961-90 for the region poleward of 62N. The plot displays the annual means (dashed blue), six-year running means (solid blue), 95% significance level (yellow), trend (dashed red), means for positive and negative LFO phases (horizontal green), and six-year running means using the 24 longest (century plus) records. Numbers at the bottom of the panel denote the number of stations used for averaging.
Arctic (green) and northern-hemispheric ([Jones et al., 1999], red) SAT trends (degC/year) (solid lines) and their 95% significance levels (dashed lines) computed from SAT time series, ranging from a 17 year (1985-2001) to the full record length (1875-2001), with 1 year increments. Arctic trends show enhanced oscillatory behavior resulting from incomplete sampling of positive and negative LFO phases.
(Left) Time series of August ice-extent anomalies (x1000km^2) in four arctic seas. (Right) Time series of annual maximum fast-ice thickness anomalies (cm) at five locations. The plot shows annual means (dotted), six-year running means (solid), and linear trends at the quoted 95% level (dashed).
Last modified: May 06, 2004. 14:57:03 pm