Preserving multicentennial climate variability in long tree-ring records is critically important for reconstructing the full range of temperature variability over the past 1000 years. This allows the putative "Medieval Warm Period" (MWP) to be described and to be compared with 20th-century warming in modeling and attribution studies. We demonstrate that carefully selected tree-ring chronologies from 14 sites in the Northern Hemisphere (NH) extratropics can preserve such coherent large-scale, multicentennial temperature trends if proper methods of analysis are used. In addition, we show that the average of these chronologies supports the large-scale occurrence of the MWP over the NH extratropics.

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Nature © Macmillan Publishers Ltd 1998 8 letters to nature 450 NATURE | VOL 393 | 4 JUNE 1998emissions. Gupta and Cicerone 16 found that in a two-dimensional model simulation where 10% of CH 4 , CO and NO x emissions were shifted from northern mid-latitudes to the northern tropics, the CH 4 steady-state lifetime decreased by 1.4%.The main sources of CH 4 to the atmosphere have been identified 14 , but emission rates from individual sources are not known well enough to quantify trends in sources. Despite this, evidence exists that some regional sources may be changing. In the states of the former Soviet Union, production of natural gas, oil and coal have all declined during the late 1980s and early 1990s (http:// www.bp.com/bpstats), and the fraction of gas that is vented or flared has also decreased 17 . This would lead to decreased emissions from the high northern latitudes. If this speculative scenario is true, emissions from the tropics must have increased to maintain a nearly constant global source strength.Our conclusions that CH 4 sources have been nearly constant from 1984 to 1996 has important implications for policy. If CH 4 sources and [OH] remain at 1996 levels, the globally averaged CH 4 mole fraction will slowly increase to ϳ1,800 nmol mol −1 over the next two decades, and then stabilize. As the imbalance between CH 4 sources and sinks is currently very small, policies that reduce CH 4 emissions by a few per cent would nearly ensure a decrease of CH 4 levels in the atmosphere 18 . We point out, however, that quantifying small, longterm trends in specific CH 4 source strengths remains impossible, so the future atmospheric burden of CH 4 cannot be predicted with certainty 19 . A better understanding of the CH 4 budget, and how it is changing with time, is still needed to predict more accurately the future role of CH 4 in climate change. Ⅺ

We investigated the response of conifer trees in northern Eurasia to climate change and increasing CO 2 over the last century by measuring the carbon isotope ratio in tree rings. Samples from Larix, Pinus and Picea trees growing at 26 high-latitude sites (59-711N) from Norway to Eastern Siberia were analysed. When comparing the periods 1861-1890 and 1961-1990, the isotope discrimination and the ratio of the intercellular to ambient CO 2 concentration (c i /c a ) remained constant for trees growing in mild oceanic climate and under extremely cold and dry continental conditions. This shows a strong coordination of gas-exchange processes, consisting in a biochemical acclimation and a reduction of the stomatal conductance. The correlation for c i /c a between the two investigated periods was particularly strong for Larix (r 2 5 0.90) and Pinus (r 2 5 0.94), but less pronounced for Picea (r 2 5 0.47). Constant c i /c a under increasing CO 2 in the atmosphere resulted in improved intrinsic water-use efficiency (W i ), the amount of water loss at the leaf level per unit carbon gain. We found that 125 out of 126 trees showed increasing W i from 1861 to 1890 to 1961 to 1990, with an average improvement of 19.2 AE 0.9% (mean AE SE). The adaptation in gas exchange and reduced transpiration of trees growing in this region must have had a strong impact on the water and energy budget, resulting in a drier and warmer surface air layer today than would exist without this vegetation-climate feedback.

Abstract. We describe new reconstructions of northern extratropical summer temperatures for nine subcontinental-scale regions and a composite series representing quasi "Northern Hemisphere" temperature change over the last 600 years. These series are based on tree ring density data that have been processed using a novel statistical technique (age band decomposition) designed to preserve greater long-timescale variability than in previous analyses. We provide time-dependent and timescale-dependent uncertainty estimates for all of the reconstructions. The new regional estimates are generally cooler in almost all precalibration periods, compared to estimates obtained using earlier processing methods, particularly during the 17th century. One exception is the reconstruction for northern Siberia, where 15th century summers are now estimated to be warmer than those observed in the 20th century. In producing a new Northern Hemisphere series we demonstrate the sensitivity of the results to the methodology used once the number of regions with data, and the reliability of each regional series, begins to decrease. We compare our new hemisphere series to other published large-regional temperature histories, most of which lie within the 1 • confidence band of our estimates over most of the last 600 years. The 20th century is clearly shown by all of the palaeoseries composites to be the warmest during this period.

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Tree-ring data have been used to reconstruct the mean summer (April-August) temperature of northern Fennoscandia for each year from AD 500 to the present. Summer temperatures have fluctuated markedly on annual, decadal and century timescales. There is little evidence for the existence of a Medieval Warm Epoch, and the Little Ice Age seems to be confined to the relatively short period between 1570 and 1650. This challenges the popular idea that these events were the major climate excursions of the first millennium, occurring synchronously throughout Europe in all seasons. An analysis of past warming trends suggests that any summer warming induced by greenhouse gases may not be detectable in this region until after 2030