Earth Gauge » Atmosphere

Climate Fact: Jet Stream Trends

administrator — Wed, 08 Oct 2008 21:51:19 +0000

At the tropopause (the point in altitude where the lowest part of Earth’s atmosphere, the turbulent troposphere, transitions into the more stable stratosphere), which is located at about nine miles up, bands of 200 mile-per-hour air currents flow around the world while periodically meandering north and south. These air currents are known as jet streams. There are two types of jet streams, the more powerful polar jets, which generally flow between 35 and 65 degrees in latitude (depending on the season) and the less powerful subtropical jet streams, which are located at the poleward edges of the Hadley Cells, or the points where the air that rises from the Equatorial Region descends towards that surface. These jet streams control storm tracks as well as storm frequency and intensity. Their presence also works to suppress hurricane formation in the area over which they flow. Over the past three decades, their behavior has changed. As the edges of the Hadley Cells have expanded towards the poles, so has the Southern Hemisphere subtropical jet stream, which has weakened during this period. The Northern Hemisphere polar jet stream has weakened as well, and has also moved closer to the North Pole. In contrast, the Southern Hemisphere polar jet stream appears to be strengthening, especially during June, July, and August, and it has, on average, moved close to the South Pole. All of the jet streams have risen in altitude during this period, which corresponds to a general warming of the troposphere.

Seasons: Winter, Spring, Summer, Fall

Source: Archer, CL and Caldeira, K. “Historical trends in the jet streams.” Geophysical Research Letters 35 (2008): L08803

Climate Fact: Carbon Catch in the Amazon

administrator — Wed, 08 Oct 2008 19:26:48 +0000

Since at least the late 1970’s, trees in the Amazon have been growing faster despite the fact that climbing vines (lianas), which grow on trees as parasites and sap their energy, have also increased in number and spatial extent. This growth means that the rate at which each hectare (2.47 acres) in the Amazon takes carbon dioxide (CO2) out of the atmosphere has been increasing by about one metric ton (2200 pounds) per year since 1975, a trend can be largely explained by an increase in available atmospheric CO2, of which concentrations have grown from 280 parts per million in pre-industrial times to around 386 parts per million today. Another factor that helps to explain this faster growth is an increase in sunlight over the region during the last 30 years. More sunlight means more available energy for photosynthesis, the process through which plants use energy from the sun to convert atmospheric carbon into chemical energy and living matter. Increases in nitrogen and phosphorous (two key plant nutrients) deposition on the area, as a result of more burning and more dust blowing over the Atlantic from the Sahara, may also be contributing to the increased plant growth.

Seasons: Winter, Spring, Summer, Fall

Sources: Baker, TR et al. “Increasing biomass in Amazonian forest plots.” Philosophical Transactions of the Royal Society of London B 359 (2004): 353-365 and Phillips, OL et al. “Changes in the Carbon Balance of Tropical Forests: Evidence from Long-Term Plots.” Science 282 (1998): 439-442 and Phillips, OL et al. “Pattern and process in Amazon tree turnover, 1976-2001.” Philosophical Transactions of the Royal Society of London B 359 (2004): 381-407.

Climate Fact: Higher Lows

administrator — Wed, 08 Oct 2008 19:18:43 +0000

Getting a break from exposure to hot temperatures is important for preventing heat related illnesses. While people usually associate extreme daytime temperatures with heat stroke, if it does not cool sufficiently during the night, the body will not get a break from the heat. In North America over the last 50 years, average nighttime low temperatures have risen faster than average daytime high temperatures. There has been a 50 percent increase in the number of unusually warm nights, and nights with temperatures that would have fallen into the top tenth percentile during the 1950’s now fall into the top fifteenth percentile. Almost all of this increase has happened since 1975.

Season: Summer

Source: United States. Climate Change Science Program. Weather and Climate Extremes in a Changing Climate. Synthesis Assessment Product 3.3: GPO. 2008.

Climate Fact: Pinatubo and Photosynthesis

administrator — Wed, 08 Oct 2008 19:15:15 +0000

The June 15, 1991 eruption of Mt. Pinatubo, which sits on the island of Luzon in the Philippines, injected between 31 and 44 billion pounds of sulfur dioxide (SO2) into Earth’s stratosphere (the second layer of the Earth’s atmosphere). This layer of SO2 circled the globe in about three weeks, and by the end of 1991 had made its way to the Polar Regions. The layer formed an “aerosol envelope,” which absorbed incoming solar radiation, making the stratosphere warmer than average (by about seven degrees Fahrenheit in the tropical stratosphere), and the surface cooler than average (by about one degree Fahrenheit). Another effect of this eruption was a reduction in the rate of atmospheric carbon dioxide (CO2) increase. This effect has been attributed to a general planetary increase in the rate of photosynthesis, or an increase in the rate at which plants use energy from the sun to convert atmospheric CO2 into chemical energy and plant matter. While more sunlight is generally conducive to plant growth, plants use “diffuse” radiation more effectively than they do direct radiation. The sulfur dioxide in the stratosphere helped to make the sunlight that reached the plants at Earth’s surface ideal for plant growth. At one forest site, total photosynthesis in 1992 (the year when sulfur concentrations in the stratosphere were at their peak) was 21 percent higher than it was under average conditions. As the stratosphere began to clear, the rate of photosynthesis, and thus plants’ uptake of carbon dioxide, began to decline. In 1993, the photosynthetic rate was six percent above average, and in 1994 it was three percent above average.

Seasons: Winter, Spring, Summer, Fall

Sources: Gu, Lianhong et al. “Response of a Deciduous Forest to the Mount Pinatubo Eruption: Enhanced Photosynthesis.” Science 299 (2003): 2035-2038 and Wolfe, Jason. “Volcanoes and Climate Change.” NASA Earth Observatory 5 September 2000. 16 July 2008 <http://earthobservatory.nasa.gov/Study/Volcano/>

Climate Fact: Tropical CAPE

administrator — Wed, 08 Oct 2008 17:42:41 +0000

Convective available potential energy (CAPE) is a measure of how much energy is available for storm development (CAPE is measured by the number of joules present in a kilogram of air). Generally, the hotter and more humid conditions are, the more CAPE is present. A collection of atmospheric conditions, including some CAPE, are necessary for a storm to develop. Thus, while CAPE does not tell you whether there will be a storm or not, it does tell you how severe a storm that does develop is likely to be. Since the late 1950’s, the average amount of CAPE in the tropics has been growing at a rate of 86 joules per kilogram per decade, or six percent per decade. This trend has been the most pronounced in the western tropical Pacific.

Seasons: Winter, Spring, Summer, Fall

Source: Gettelman, A. “Multidecadal trends in tropical convective available potential energy.” Journal of Geophysical Research 107 (2002): ACL 17.

Climate Fact: Winter Weather and the North Atlantic Oscillation (Chicago, IL)

administrator — Wed, 05 Mar 2008 22:48:07 +0000

The North Atlantic Oscillation (NAO) is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar? air masses from invading the lower latitudes. This tends to keep winter weather in the mid- latitudes relatively mild and reduce the occurrence of below average winter temperatures in the United States.? In Chicago, for example, there are on average three times as many days each year when the temperature drops below zero degrees Fahrenheit during negative phases, versus positive phases.? Over the last thirty years, the index has been predominately positive. The index is currently hovering around neutral, and it has been mostly positive this winter.

Season: Winter

Climate Fact: Winter Weather and the North Atlantic Oscillation (Boston, MA)

administrator — Wed, 05 Mar 2008 22:46:08 +0000

The North Atlantic Oscillation (NAO), which is part of a large system known as the Arctic Oscillation, is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar air masses from invading the lower latitudes. This tends to keep winter weather in the Northeast relatively mild, but it also keeps the cold polar air in Canada’s Maritime Provinces and Quebec, making winters in these regions more severe. Over the last thirty years, the index has been predominately positive, which has contributed to the increased winter temperatures in the northeastern U.S. during this period. The index is currently positive, and has been mostly positive this winter.

Season: Winter

Sources: Thompson, David W.J. “Regional Climate Impacts of the Northern Hemisphere Annular Mode.” Science 293, 85 (2001) and National Oceanic and Atmospheric Administration: Climate Prediction Center. North Atlantic Oscillation. Accessed Online 19 February 2008 and Wettstein, JJ and Mearns, LO. “The Influence of the North Atlantic-Arctic Oscillation on Mean, Variance, and Extremes of Temperature in the Northeastern United States and Canada.” Journal of Climate 15, 3586 (2002)

Climate Fact: Winter Weather and the North Atlantic Oscillation (Atlanta, GA)

administrator — Wed, 05 Mar 2008 22:43:42 +0000

The North Atlantic Oscillation (NAO) is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar air masses from invading the lower latitudes. This tends to keep winter weather in the mid- latitudes relatively mild and reduce the occurrence of below average winter temperatures in the United States. In Atlanta, for example, there are on average five times as many days each year when trace snow falls occur during negative phases, versus positive phases. Over the last thirty years, the index has been predominately positive. The index is currently positive, and has been mostly positive this winter.

Season: Winter

Climate Fact: ENSO and Carbon Concentrations

administrator — Wed, 05 Mar 2008 22:35:19 +0000

Atmospheric carbon dioxide (CO₂)levels have risen from 280 parts per million in pre-industrial times to around 385 parts per million today. The rate of this rise has varied from year to year. The two phases of the El Niño Southern Oscillation Cycle (ENSO), El Niño (positive) and La Niña (negative), appear to have a significant influence on this variation. The influence that the cycle has on temperature and rainfall in the tropics appears to in turn affect the rates of vegetation growth, which takes CO₂ out of the atmosphere, and plant respiration, which? takes place? when plants use the energy reserves that they stored during years that were more conducive to growth; respiration releases CO₂ into the atmosphere. ENSO’s influence seems to be especially strong in southeastern Asia and the western Amazon; during warm and dry El Niño years, plants in these regions grow at a slower rate, there are more fires, and plants consume more of their stored energy. During cool and wet La Niña phases, there is more plant growth, fires are rarer, and plants are storing more energy reserves than they are consuming. The influence that ENSO phases have on tropical forest conditions is thought to largely account for the variation in annual growth rates in atmospheric CO₂concentrations. In the same ENSO cycle, these rates can vary as much as 225 percent between the El Niño and La Niña phases.

Seasons: Winter, Spring, Summer, Fall

Source: Heimann, Martin and Reichstein, Markus. “Terrestrial ecosystem carbon dynamics and climate feedbacks.” Nature (2008), vol 451. pp. 289-292

Climate Fact: Longleaf Pines and Carbon Dioxide

administrator — Tue, 15 Jan 2008 18:23:42 +0000

Prior to European settlement, Longleaf Pine forests covered a 140 million square mile area that runs along the Atlantic and Gulf Coasts from Southern Virginia to Texas. Frequent, low-intensity fires, which traditionally happened every two to four years and would sweep across the forest floor like a broom without killing the Pine trees themselves, keeping the forests relatively “open.” This “openness” allowed ample sunlight to reach the forest floor, and ground dwelling bunchgrass and wildflower species, many of which are endangered or threatened today, thrived. These ecosystems also support endangered animal species such as the Gopher Tortoise and Red Cockaded Woodpecker. While fire suppression and conversion of much of the Southeast to agricultural and urban uses mean that the Longleaf Pine forests only cover about four percent of their original range, the rise in atmospheric carbon dioxide (CO₂) levels (from 280 parts per million in pre-industrial times to around 385 parts per million today) may be beginning to alter the Longleaf Pine forests that do remain. Traditionally, Longleaf Pines account for about 76 percent of the total mass in a forest, and forest floor species such as wiregrass, rattlebox, and butterfly weed account for about 19 percent. Under elevated carbon dioxide (CO₂) levels, however, Longleaf Pines account for 88 percent of these forests’ biomass while the forest floor species only account for eight percent. This has implications for the future resilience and biodiversity of these forests.

Seasons: Winter, Spring, Summer, Fall

Sources: McGinnis, Laura. “Elevated Carbon Dioxide Has Uneven Influence on Longleaf Communities.” United States Department of Agriculture: Agricultural Research Service News and Events. 11 December 2007. Accessed Online 3 January 2008 < http://www.ars.usda.gov/is/pr/2007/071211.htm> and Hilton, Jarel. “Biological Diversity in the Longleaf Pine Ecosystem.” Alabama’s Treasured Forests: Fall 1999. Accessed Online 3 January 2008 www.forestry.state.al.us/publication/TF_publications/tffall99/biological_diversity_in_the_longleaf_pine_ecosystem.pdf and Auburn University: The Longleaf Alliance, 2002.? “The Longleaf Pine Fireforest.” Accessed Online 3 January 2008 http://www.auburn.edu/academic/forestry_wildlife/longleafalliance/ecosystem/ecosystem.htm

Climate Fact: Hadley Cell Expansion

administrator — Tue, 15 Jan 2008 17:40:11 +0000

Map makers (also known as cartographers) and astronomers site the tropics as the region between the latitudes of 23.5 degrees north and 23.5 degrees south, or the two points where the sun is directly overhead on the summer and winters solstices respectively. Climatologists define the tropics slightly differently, with the basis of the definition relying on temperature and precipitation patterns. In tropical climates, there is little seasonal or even day-to-day variation in temperature and rainfall. At the sub-solar point, or the point where the sun’s radiation is most intense, hot and humid air rises and then moves poleward in both directions. As it moves, the moisture in this air falls as rain. Once the air mass has literally “run out of steam,” it descends and then moves back toward the sub-solar point. The regions where the air mass descends are noticeably drier. The region over which the rain falls can be considered the “tropical belt,” and the drier areas are the belt’s edges. Inside the belt, surface winds generally blow from east to west, whereas on the outside they blow from west to east. This system of circulation is known as the Hadley Circulation, and the region where this system controls the weather is one way to define the tropics. Since 1979, the area over which the Hadley Circulation is present has been expanding and is now between two and 4.5 degrees wider than it was in the 1970’s.

Seasons: Winter, Spring, Summer, Fall

Source: Seidel, D.J., Q, Fu, W.J. Randel and T.J. Reichler, Widening of the tropical belt in a changing climate. Nature Geoscience, doi:10.1038/ngeo.2007.38 (published online: 2 December 2007)

Climate Fact: Carbon Dioxide Concentrations and Leaf Drop

administrator — Fri, 07 Dec 2007 22:56:54 +0000

Atmospheric carbon dioxide (CO₂) levels have risen from about 280 parts per million in pre-industrial times to around 390 parts per million today. During winter in temperate deciduous forests, the days are too cold and short for trees to efficiently make food, and as a result, the trees are dormant during this time of year. During autumn, trees prepare for winter by halting the production of chlorophyll, or the green chemical that plants use to produce food. When they do this, the yellows and oranges that have been hidden in the leaves all along become visible, and in some species, leftover sugars cause the leaves to turn red. According to recent experiments that grew three species of North American deciduous trees in CO₂ enriched environments, the more CO₂ there is in the atmosphere, the later in the year trees lose their green color.?

Season: Fall

Source: Karnosky, DF et al. “Rising atmospheric CO2 explains 26-52% of the recent delay in autumnal senescence in important forest and crop species.” Plants and Environment Lab: University of Southampton (2007)

Climate Fact: Boiling Point

administrator — Fri, 07 Dec 2007 22:35:51 +0000

In the last fifty years, the temperature of the upper 3000 meters of the ocean (which is about 70 percent of the World’s ocean water), rose by 0.07 degrees Fahrenheit. While this number may seem small, the same amount of energy it would take to raise the? world’s ocean heat content by just 0.18 degrees would be enough to raise the average global atmospheric temperature to the boiling point of water, or 212 degrees! For further comparison, the Earth’s current temperature is about 59 degrees.

Seasons: Winter, Spring, Summer, Fall

Sources: Levitus, S. et al. “Warming of the world ocean, 1955-2003.” Geophysical Research Letters, vol. 32 (2005) L02604, doi: 10.1029/2004GL01592 and Miles, Edward. Multiple Stresses, Thresholds, and Ocean Acidification. Cannon House Office Building, Washington, DC. 20 September 2007.

Climate Fact: Climbing Vines and Carbon

administrator — Fri, 07 Dec 2007 22:15:03 +0000

Woody climbing vines, which in the Amazon Rainforest are known as lianas, account for only about five percent of the total plant mass in the rainforest, but up to 40 percent of the total leaf production. Lianas can produce so many leaves because instead of using energy to build hard, lignin rich tree trunks, they simply “free load” off the trees that are already there!? Lianas harm the trees that they attach to and use as ladders, however, and are considered parasites. Trees that “host” the lianas grow at slower rates and have on average shorter lives than unaffected trees. Experiments have shown that the more carbon dioxide (CO₂) there is in the atmosphere, the faster woody climbing vines grow compared to the trees that host them. CO₂levels have risen from 280 parts per million in pre-industrial times to around 390 parts per million today. This is the most likely explanation for the fact that lianas are now increasing in dominance throughout the Amazon Rainforest at a rate of 1.7-4.6 percent a year.

Seasons: Winter, Spring, Summer, Fall?

Source: Phillips, OL et al. “Increasing dominance of large lianas in Amazonian forests.” Nature, vol. 418 (2002) 770-774

Climate Fact: Net Primary Productivity

administrator — Mon, 05 Nov 2007 22:48:37 +0000

Terrestrial Net Primary Production (NPP) is the total amount of vegetation that the Earth’s land plants make by using sunlight, carbon dioxide (CO₂), and water. Total NPP is influenced by climatic conditions such as atmospheric CO₂ levels, rainfall amounts and patterns, cloud cover, and temperature. Factors that limit NPP include longer winters, increases in cloud cover, and decreases in water availability, while shorter winters, less cloud cover, and more available water increase NPP. The same climate trend may affect NPP differently in different regions. For example, less rain in a region that already receives ample rainfall is likely to coincide with a decrease in cloud cover and more sunlight, which may increase NPP in that particular region. However, in arid regions, less rainfall is likely to decrease NPP despite increases in sunlight. Between 1982 and 1999, total terrestrial NPP increased by over six percent, and higher CO₂levels have been cited as a major factor in this increase. Almost half of this increase (42 percent), however, happened in the Amazon Rainforest Region. This has been attributed to changes in rainfall patterns in that Region and a resulting decrease in cloud cover and increase in sunlight. This increase in production may be only temporary, however, as these changes in rainfall patterns are likely to ultimately dry out the Amazon, which will decrease the Region’s NPP.

Seasons: Winter, Spring, Summer, Fall

Source: Nemani, RR. “Climate-Driven Increases in Global Terrestrial Net Primary Production from 1982 to 1999.” Science: 300, pp. 1560-1563 (2003).

Climate Fact: “Sticky” Southeast

administrator — Tue, 09 Oct 2007 14:55:26 +0000

The more water in the atmosphere, the less easily sweat evaporates from your body, and the harder it is to keep your body at a comfortable temperature. The dewpoint temperature is one of the best indicators of how “uncomfortable” hot weather is, and when the dewpoint temperature exceeds 65 degrees Fahrenheit, most people consider the weather to be “sticky.” In the Southeastern United States, while the average temperature during the fall has increased only slightly over the past half century, the average dewpoint temperature during the fall has been rising by about one degree Fahrenheit per decade. For the nation as a whole, the average annual dewpoint temperature has been rising by about 0.54 degrees Fahrenheit per decade.

Season: Fall

Sources: Gaffen, DC and Ross, RJ. “Climatology and Trends of U.S. Surface Humidity and Temperature.” Journal of Climate: Volume 12, Issue 3, pp. 811-828 and Roderick, ML et al. “The Cause of Decreased Pan Evaporation over the Past 50 Years.” Science 298, 1410 (2002).

Climate Fact: Jumpin’ Juniper

administrator — Tue, 09 Oct 2007 14:48:10 +0000

Just to the east of the Cascades, Ponderosa and Lodgepole Pine forests transition into the Central Oregon Prairie, or the Central Oregon Steppe. This prairie is dotted with Sagebrush, various grasses, and Western Juniper trees, which are short and compact trees known for producing Juniper berries. Juniper trees have become increasingly common over the last century or so, both because of ranching practices, which tend to remove competing vegetation and thus allow the Juniper saplings to establish themselves, and because of climate change, particularly higher levels of atmospheric carbon dioxide. Atmospheric levels of carbon dioxide (CO₂) are higher than they have been at any point in at least the last 400,000 years, and have gone from a concentration of about 280 parts per million (ppm) during in the 18th Century to over 380 ppm today. It has been found that plants respond to higher CO₂ concentrations by closing their stomata, which are the tiny openings usually found on the undersides of leaves that regulate how much gas (water, oxygen, CO₂) goes in and out of a plant. When plants close their stomata, they lose less water. This ability to reduce water loss is especially beneficial for perennial species such as the Juniper, and helps to explain why in the last thirty years, Juniper cover has doubled in areas of central Oregon, even areas that are unaffected by human activities that promote the trees’ establishment. Warmer temperatures, and thus a longer growing season, have also helped the species.

Seasons: Spring, Summer, Fall

(Source: Soule, PT et al. “Human Agency, Environmental Drivers, and Western Juniper Establishment During the Late Holocene.” Ecological Applications, 14(1), 2004, pp. 96-112.)

Climate Fact: Walker Circulation and SW Australiaâ€™s Water

administrator — Tue, 09 Oct 2007 14:40:57 +0000

The Walker Circulation is a circular belt of air that moves around the tropical Pacific. The warm moist air of the trade winds moves westward from the coast of South America. As these winds move, the air travels upward, cools, and rain falls. This drier and cooler air then moves in the upper levels of the atmosphere back towards South America, descends towards the sea, and takes the place of the warm moist air moving in the opposite direction. When the circulation is strongest, Indonesia and Australia receive plentiful rainfall. El NiÃ±o events tend to weaken this circulation, however, and a recent shift to more frequent and intense El NiÃ±o events, as well as a general warming of ocean water temperatures, has coincided with a drying of Southwestern Australia. Between 1974 and 2005, average annual inflow into the dams that supply water to the Western Australian city of Perth fell from 338 gigalitres (338 billion liters) to 114 gigaliters.

Seasons: Winter, Spring, Summer, Fall

(Source: Hopkin, Michael. “Global warming Weakens Pacific Winds.” News@nature.com 3 May 2006. Accessedn Online 18 January 2007 and Vecchi, G.A., B.J. Soden, A.T. Wittenberg, I.M. Held, A. Leetmaa, and M.J. Harrison (2006). Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature. v.441, doi:10.1038/nature04744 and Wahlquist, Asa. “El NiÃ±o Shaped by Global Warming.” The Australian. 26 September 2007. Accessed Online 26 September 2007 )

Climate Fact: Runoff Rise

administrator — Tue, 09 Oct 2007 14:06:31 +0000

Rising temperatures in many parts of the United States have caused an increase in evaporation and drying of the soil. This is especially true in the Southwestern United States where there has been a persistent drought since about 2000, despite there being no real change in the amount of rainfall. At the same time, there has been an increase in continental runoff, or the amount of water that flows from the land to the ocean. During the last forty years in North America, the amount of water that each square meter of land contributes to annual runoff has grown by over a liter, and some of this increase in runoff has been attributed to higher Carbon Dioxide (CO2) levels. Atmospheric levels of carbon dioxide (CO2) are higher than they have been at any point in the last 400,000 years, and have gone from a concentration of about 280 parts per million (ppm) during in the 18^th Century to over 380 ppm today. It has been found that plants respond to higher CO2 concentrations by closing their stomata, which are the tiny openings usually found on the undersides of leaves that regulate how much gas goes in and out of a plant. When plants close their stomata, there is less water going from the soil through the plant and into the atmosphere, meaning that more water stays in the soil and ultimately drains into the oceans. This decrease in evaporation from plants is expected to partially counteract the increase in evaporation resulting from higher temperatures, which may help to mitigate drought, but may also increase the risk of floods.

Seasons: Winter, Spring, Summer, Fall

Sources: Gedney, N et al. Detection of a direct carbon dioxide effect in continental river runoff records.” Nature: Volume 439, 835 (2006) and Betts, RA et al. “Projected increase in continental runoff due to plant responses to increasing carbon dioxide.” Nature: Volume 448, 1037 (2007).

Climate Fact: Almighty AMO (Western U.S.)

administrator — Wed, 19 Sep 2007 15:27:40 +0000

Researchers found that since the mid-1980’s, the length of the wildfire season (the period of the year when it is dry and hot enough for fires to happen) in the forests of the Western United States has grown 78 days longer as a result of earlier snowmelt and increased spring and summer temperatures. These trends, as well as a century of fire suppression and other human modifications of forest structure, help to explain why the last two years set records for the number of acres affected by wildfire. Another factor that affects wildfire behavior in the Western United States is the state of the Atlantic Multidecadal Oscillation, or the 60 year cycle in the North Atlantic, where sea-surface temperatures move between warm (positive) and cool (negative) phases. The effects of this Oscillation are felt around the World. Warm (positive) sea-surface temperatures in the North Atlantic generally correspond to drier and warmer conditions in the Western United States. Tree ring records that go back almost 500 years show that the entire Western Region consistently experiences more wildfires during positive phases of the cycle. Currently, the Oscillation is trending positively.

Season: Summer, Fall

Sources: Westerling, A.L. “Warming and Earlier Spring Influence Western U.S. Forest Wildfire Activity.” Science. 313 (18 August 2006): 940-943 and Van Bussum, Larry. “2006 Fire Season a Record Year for the Nation and NWS.” Aware. National Weather Service. January 2007and Kitzberger, T et al. Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony over western North America. Proceedings of the National Academy of Sciences: January 9, 2007, vol. 104, no. 2. pps 543-548.