Fig. 5. First temporal harmonic of the average monthly air temperatures (C). Amplitude is proportional to the length of the arrow (measured from the center of the scale/dial) while the occurrence of the maximum in time is given by its direction (edge of the scale/dial) 3114 Fig. 6. Second temporal harmonic of the average monthly air temperatures ('C). Amplitude is proportional to the length of the arrow (measured from the center of the scale/dial) while the occurrence of the maxima in time is given by its directions (edge of the scale/dial) D. R. Legates and C. J. Willmott: Mean Seasonal and Spatial Variability in Global Surface Air Temperature 19 Fig. 7. Percent of the temporal variance explained in the mean monthly surface air temperatures () by the first two harmonics. Isolines are 90.0, 99.0, and 99.9 percent. Areas where is less than 90.0 percent are unshaded while areas where r is greater than 99.9 percent are dark grey (e.g., over the central United States) tion. Willmott et al. (1985a) also used harmonics to describe seasonal variations, albeit in snow cover, soil moisture, and evapotranspiration. Following Willmott et al., the amplitude and phase of the harmonics are shown as a vector (Figs. 5 and 6). The length of each vector (arrow) represents the magnitude (note the logarithmic scale) while the direction locates the occurrence in time of the maximum. A complete discussion of harmonic analysis is given by Rayner (1971). More than 99.0 percent of the variation in mean monthly surface air temperature is accounted for by the first two harmonics. Explained variance (by the two harmonics) only falls below 90.0 percent for a few areas in the mid-latitudes of the southern hemisphere and in the tropics (Fig. 7). Other mid-latitude areas have greater than 99.9 percent of their variance explained. Another way to evaluate the goodness of fit is to consider the average magnitude of the residuals or the standard error. For air temperature, the variation left unaccounted for by the first two harmonics is less than or equal to 1.25 °C for any grid point. Amplitudes of the first harmonic increase pole ward and are larger over the continents than over the oceans at the same latitude (Fig. 5). This reflects the poleward increase in seasonality as well as the moderating influence of the oceans on temperature variation. The phase of the first harmonic approximates the time of maximum monthly air temperature - mid-July in the continental northern hemisphere and mid-January over the southern hemisphere continents. Maxima are delayed for nearly two months over the oceans. Tropical regions are dissimilarly characterized by a weak double-maxima air temperature cycle. First harmonic maxima, therefore, are small in these areas. Monsoon climates (just poleward of the tropics) exhibit a maximum one or two months before the solstice. This occurs just prior to the onset of the rainy season which cools the air considerably. Equatorial oceans have weak maxima in late March or early April that are pronounced where little precipitation falls (cf.. Legates, 1987). During this time, the Peru and Benguela currents are warmest which contributes to atmospheric warming. The second harmonic explains only about one quarter as much variance as the first (Fig. 6). In higher latitudes, particularly in the southern hemisphere, the amplitude of the second harmonic is relatively large and the maxima occur about two months prior (and four months later) than the first harmonic maxima. Dominance of the pronounced seasonal cycle and the rapid warming that occurs at the onset of summer are the probable causes. Amplitudes of the second harmonic also are quite large in the tropical deserts owing to the double-maxima air temperature cycle. Monsoon climates as well are characterized by a relatively large second-harmonic amplitude again owing to a double-maxima in the seasonal air temperature cycle. 5. Concluding Remarks A relatively dense terrestrial network of long-term monthly surface air temperature stations has been compiled from existing sources, screened for coding errors, and redundant station records have been removed. Oceanic (monthly) grid-point av. erages augmented the terrestrial measurements. Station and grid-point data then were interpolated to a 0.5° of latitude by 0.5° of longitude lattice using a sperhically-based interpolation algorithm. These interpolated data are available on magnetic tape and may be obtained by contacting the authors. Maps of the interpolated air temperature field confirm and precisely locate the higher temperatures in low latitudes and the gradients toward the poles. The large intra-annual variation in the polar regions as well as small variation in the tropics also is documented. Harmonic analysis (using a 4° of latitude by 5° of longitude subset of the 0.5° by 0.5° grid) reveals the geographic extent and timing of the mid- and high-latitude seasonal air temperature cycle. Terrestrial air temperature maxima occur approximately one month after the summer solstice while maxima are delayed by an average of two months over the oceans. A doublemaxima air temperature regime characterizes the continental subtropics. Acknowledgements Most of this work was supported by the National Aeronautics and Space Administration under grant NAG 5-853. Thoughtful suggestions from James E. Burt (University of Wisconsin), Richard Garvine (University of Delaware), and Ferris Webster (University of Delaware) are greatly appreciated. References ADND, 1965: Fitzroy Region, Queensland-Climate. Canberra: Australian Department of National Development, Resources Information and Development Branch. Baker, D. G., 1975: Effect of observation time on mean temperature estimation. J. Appl. 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Printed in Austria STATEMENT OF ADAM MARKHAM, EXECUTIVE DIRECTOR, CLEAN AIR-COOL PLANET, PORTSMOUTH, NH Good morning Mr. Chairman, and members of the committee. My name is Adam Markham and I am the executive director of Clean Air-Cool Planet, a small nonprofit working to achieve reductions of greenhouse gas emissions in the Northeast. Thank you for inviting me here today to talk about likely impacts of continued climatic change. New England is coming to end of what will almost certainly be the warmest winter on record, and much of the region has been in the grip of severe or extreme drought for many months. These individual weather events are not, in themselves, indicators of climate change but they are providing a taste of what climate change might bring. New Hampshire is currently experiencing the second worst drought in more than 100 years and Maine's last 12 months were the driest on record. Lake Winnipesaukee is at its lowest level in a generation, wells are running dry, and concerns are being raised about hydroelectric power shortages, fish populations and forest fire risk. As with the rest of the country, we are experiencing a long-term warming trend. On average, New England has warmed by 0.7 °F since 1895. Winters have warmed more than summers, and the greatest warming has been in New Hampshire, Vermont and Rhode Island. Annual precipitation for the region as a whole has increased, especially in southern New England where the change has been more than 25 percent over the last century. More rain is falling in intense storms than in the past. On the other hand, there has been a significant decrease (15 percent) in snowfall in northern New England since 1953. Snow is lying on the ground 7 days less than it was 50 years ago and the ice comes off lakes a few days earlier now than 100 years ago. Other documented indicators of a shorter winter include progressively earlier flowering of lilacs and the fact that frogs have advanced their spring calling by several weeks. The New England Regional Assessment (NERA), which was carried out under the auspices of the U.S. Global Change Research Program and coordinated by Dr. Barrett Rock of the University of New Hampshire, was published in September 2001. Four years in the making, the report reviewed some of the risks associated with continued global warming. The warming scenarios described in the report suggest a likely 6-10 °F warming over the next century. In crude terms, such a change would result in Boston getting the climate of Richmond, VA in the best case, and that of Atlanta, GA in the worst case. Either way, the climate of New England would be irreversibly transformed with far-reaching and negative, economic and environmental impacts. SUGAR MAPLE Let me start by describing the threat to one of the icons of New England culture, and one that I know is close to Chairman Jeffords' heart-the sugar maple. According to all credible forest models, the sugar maple is one of the tree species most sensitive to warming temperatures. Business as usual emissions scenarios are almost certain to eventually drive the sugar maple northwards out of New England entirely. Even before that happens climate change will start to take a toll. New England and New York produce approximately 75 percent of the maple syrup produced in the U.S. today. U.S. maple syrup production is worth more than $30 million annually. For Vermont, it is a more than $100 million industry with over 2,000 mainly family owned sugar producers. Many of these families have been careful stewards of these forests for generations and they have a strong interest in the legacy that is passed to their children and grandchildren. Maple trees take decades to mature and new stands are planted for the benefit of future generations. According to NERA this heritage and industry "may be irreparably altered under a changing climate". There are indications that sugar production tends to be better in colder years, and it is established that droughts during the growing season adversely affect production in subsequent years. For example, sugarmakers expect to see impacts of the current drought, which started last summer, in production numbers for this current season. There is a very short time in the year when conditions are right for sugar production. Sap generally flows during late February and early March. Sugar bushes need a prolonged period of temperatures below 25 °F to convert starch to sucrose and to get high sugar content in the sap. A freeze/thaw cycle of cold nights and warm days (above 38-40 °F) is required to get the sap moving. When the nights no longer freeze the season is over. |