Rains in East Africa primarily fall during the long rains (March through May) and the short rains (October through December). Understanding how climate and climate change influence these rains is particularly important for predicting drought in the region.  Conditions in the tropical Pacific affect weather throughout the world and the tendency for the tropical Pacific to persist in either cool La Niña phases or warm El Niño phases enable forecasters to predict whether seasonal temperatures or precipitation levels will be above or below average. In addition to local Indian Ocean sea surface temperatures, the short rains in East Africa are strongly influenced by conditions in the tropical Pacific, with La Niña phases leading to reduced precipitation. How the tropical Pacific affects the long rain season, on the other hand, is harder to identify. Recent work shows that since 1999, the long rains have been consistently weak and unable to compensate for reduced short rain precipitation during La Niña phases, leading to drought conditions like those experienced in 2011. Also since 1999, there has been a pattern of consistently elevated sea surface temperatures in the western tropical Pacific and consistently high rainfall levels in that region. The tendency of ocean surface temperatures to remain in particularly warm or cool conditions for relatively long periods of time, coupled with what is known about the atmospheric links between the western tropical Pacific and East Africa, enables forecasters to better understand and predict droughts there.

Seasons: Winter, Spring, Summer, Fall

Source: Lynn, B and DeWiltt, DG. “A recent and abrupt decline in the East African long rains.” Geophysical Research Letters 39 (2012): L02702. 

Trivia Question: 20,000 years ago, when Earth’s climate was much cooler and a massive ice sheet extended form the Arctic south all the way to the Columbia River, the Great Salt Lake was…

a) larger than it is today.
b) smaller than it is today.
c) unchanged from where it is today.

The correct answer is a. The Great Salt Lake is a remnant of Lake Bonneville, which was several times larger 20,000 years ago. Lake Bonneville was once as large as Lake Michigan, but shrunk over the past 20,000 years, leaving behind the Great Salt Lake Desert and the Great Salt Lake. There are many salt flats and salt deserts across the Intermountain West, which are indications of the wetter climate and greater lake coverage that existed in the region during past glacial periods. Similar processes happened elsewhere around the world, including in the Sahara Desert. Ancient, glacial era lake beds across the world constitute Earth’s major atmospheric dust sources.

Seasons: Winter, Spring, Summer, Fall

Sources: Munroe, JS et al. “Latest Pleistocene advance of alpine glaciers in the southwestern Uinta Mountains, Utah, USA: Evidence for the influence of local moisture sources.” Geology 34 (2006): 841-844 and Dyke, AS et al. “The Laurentide and Innuitian ice sheets during the Last Glacial Maximum.” Quaternary Science Reviews 21 (2002): 9-31.

Trivia Question: Does the world have more wet or dry areas than it did in 1950?

a) Wet areas
b) Dry areas

The correct answer is b. The Earth’s land surface is drier overall than it was in the early part of the 20th century. Dry area coverage has been growing at a rate of 1.74 percent per decade since the early 1980’s. The United States has been an exception to this general rule: despite periods of severe drought, particularly in parts of the West, wet area coverage has been increasing by about one percent each decade.

Seasons: Winter, Spring, Summer, Fall

Source: Dai, A. “Characteristics and trends in various forms of the Palmer Drought Severity Index during 1900-2008.” Journal of Geophysical Research 116 (2011): D12115.

Dust suspended in the air and smoke from fires make up most of the aerosol concentrations found in the air around us. Aerosols affect how much sunlight reaches the Earth’s surface and how clouds form, which means they can have a strong influence over regional precipitation. Aerosols also link different parts of the world: winds can carry aerosols from fires and dust storms far away, even across oceans, affecting the climates of remote regions. This is particularly true for the Amazon rainforest. Dust storms in the Sahara Desert, combined with the smoke that is generated during Central Africa’s dry season when fires are common, are carried by northeasterly trade winds across the Atlantic to the Amazon. This transport is most common from January to May and takes approximately ten days to reach South America from Africa. The January through May period is the wet season in most of the Amazon, when there are few fires and dust generation is at a minimum. During this time, aerosols from Africa impact air in the Amazon – they are present in about one-third of aerosol measurements taken across the Amazon region.  Aerosols are considered a wild card in Amazonian weather and climate prediction. Variations in aerosol concentrations can make the difference between rainfall occurring or not. Generally, higher aerosol concentrations mean less rainfall. African dust does, however, provide a fertilizer for the soils of the Amazon, helping plant growth.

Seasons: Winter, Spring, Summer, Fall

Source: Baars, H. et al. “Further evidence for significant smoke transport from Africa to Amazonia.” Geophysical Research Letters 38 (2011): L20802.

Every place in the world has its own climate with its own average temperature and precipitation levels. Some places around the world, such as most equatorial rainforests, experience little seasonal variation and are hot and wet throughout the year. Locations closer to the poles have more seasonal climates. The Northeast United States, for example, has rainfall evenly spread throughout the year, but the summer months are much warmer than the winter months. In some locations in the tropics, temperatures don’t change much throughout the year, but there are defined wet and dry seasons. Similarly, from year to year some places have little rainfall variance while other locations can be considerably wetter or drier. Such inter-annual variability in rainfall can be particularly pronounced in parts of Amazon. Inter-annual rainfall variability in the Amazon is driven by surface temperature shifts every few years in the waters of the equatorial Pacific Ocean, and shifts on longer time scales in the tropical and subtropical Atlantic Ocean. These water temperature shifts move winds, affecting rainfall patterns on land. If it was not for the plants of the Amazon, however, this variability would be even greater. Wetter years, or years with wetter wet seasons and/or shorter dry seasons, are years when vegetation flourishes. More vegetation means more evapotranspiration, the movement of moisture from the soil through plants to the atmosphere. More evapotranspiration means more rainfall. This increase in rainfall helps to keep the wet conditions going into the next year, even if other factors would favor drier conditions. Drier years tend to stress vegetation, leading to less evapotranspiration and less rainfall, which perpetuates these dry conditions into the following year. This means that years following severe droughts, such as the “once-in-a-century” 2010 Amazon drought, may be more susceptible to further droughts. It also means that there is more year-to-year continuity of rainfall levels than there would otherwise be.

Seasons: Winter, Spring, Summer, Fall

Source: Wang, G et al. “Vegetation dynamics contributes to the multi-decadal variability of precipitation in the Amazon region.” Geophysical Research Letters 38 (2011): L19703.

The glaciers that make the peaks of the Western United States white year round are located almost exclusively within National Forests and National Parks. In addition to providing scenery and tourist attractions, these glaciers partially melt during the summer months, feeding streams and rivers that bring waters to cities and farms in the West. In fact, most of the West’s water comes from mountain snowpack or glaciers. As long as the glaciers accumulate enough snow during the wet and cold winter months to offset the summer melt, they persist. When temperatures warm however, as they have over the past century, summertime melt exceeds winter freeze, and the glaciers shrink in size. Warmer temperatures over the past century have resulted in melting glaciers across the National Parks and Forests of the West:

• North Cascades National Park has 40 percent less glacial mass than it did in 1850, and Lewis Glacier disappeared in 1990. Streamflow in the watershed fed by Lewis subsequently declined by 40 percent.
• In Alaska’s Glacier Bay National Park, Muir Glacier, one of the Park’s best recognized, is a mile shorter and 600 feet thinner than it was in 1980.
• The glaciers of Glacier National Park have been some of the hardest hit in the West: Today, about 27 percent of the area of Glacier National Park that was covered by glaciers in 1850 is still covered. The number of glaciers has been cut from 150 to 37.
• In the past 110 years, the seven largest of Mt. Hood’s eleven glaciers in Oregon’s Mt. Hood National Forest have each lost more than 30 percent of their masses. Sandy Glacier, which faces downtown Portland, has retreated by 60 percent during this period.

Sources: “Most Alaskan Glaciers Retreating, Thinning, and Stagnating, Says Major USGS Report: Press Release.” U.S.G.S. Newsroom. 2008. U.S. Department of the Interior, U.S. Geological Survey: Office of Communication. 6 October 2008 < http://www.usgs.gov/newsroom/article.asp?ID=2033> and United States. National Park Service. Climate Monitoring in Glacier Bay National Park and Preserve: Capturing Climate Change Indicators. 2007 Annual Report. Washington, GPO, 2007. Accessed Online 3 November 2008 and Sources: United States Geologic Survey. “Fifty-Year Record of Glacier Change Reveals Shifting Climate in the Pacific Northwest and Alaska, USA.” 6 July 2009. Accessed Online 7 August 2009 and The National Park Service. “North Cascades National Park Complex: Glacial Monitoring Program.” Accessed Online 10 July 2007 and Global Glacier Retreat Project. Nichols College. Accessed Online 5 July 2007 and Jackson, K.M. and Fountain, A.G. “Spatial and morphological change on Eliot Glacier, Mount Hood, Oregon, USA.” Annals of Glaciology 22 (2007): 222-226 and Fagre, DB. “Adapting to the Reality of Climate Change at Glacier National Park, Montana, USA.” Proceedings I Conferencia Cambio Climático 2005.

• Tree Cover: As temperatures have warmed since 1970, tree cover in areas that had not been burned or logged has increased by 40 percent.
• Salamander Decline: A two degree rise in average summertime temperatures means more water is evaporating from the Park, which has impacted local salamander populations. Many of the Yellowstone’s formerly permanent ponds are going dry in the summer and many seasonal ponds are gone. Whereas there were 43 ponds within the park that supported salamanders in the early 1990s, there are fewer than 21 today.
• Geysers: About half of Earth’s geysers are found in Yellowstone. How frequently these geysers erupt is connected to rainfall and groundwater levels, with drier periods having fewer eruptions. A good proxy for groundwater levels is the annual discharge of the Madison River, which has dropped by about 15 percent since 1970.

Seasons: Winter, Spring, Summer, Fall

Sources: Hurwitz, S et al. “Climate-induced variations of geyser periodicity in Yellowstone National Park, USA.”  Geology 36 (2008): 451-454 and “U.S. Temperature and Precipitation Trends.” U.S. National Oceanic and Atmospheric Administration (NOAA): Climate Prediction Center. 5 January 2005. Accessed Online 31 October 2008 and McMenamin, SK et al. “Climatic change and wetland desiccation cause amphibian decline in Yellowstone National Park.” PNAS 105 (43). Accessed Online 31 October 2008 and Science Daily. “Global Warming is Killing Frogs and Salamanders in Yellowstone Park, Researchers Say.” 29 October 2008. Accessed Online 31 October 2008 and Hansen, Andrew. “Conifer Cover Increase in the Greater Yellowstone Ecosystem: Frequency, Rates, and Spatial Variation.” Ecosystems 10 (2007): 204-216.

The correct answer is true. Bacteria are single-celled organisms that are found on every continent, at the bottom of the ocean and as high as 50 miles in the atmosphere. Each year, between 90 million and four billion pounds of bacteria travel from the Earth’s surface into the air around us. While most of these bacteria sink down to the surface within days or weeks, winds sometimes carry them into circulation patterns that keep them aloft for years. Many bacteria return to Earth during rainfall, with both living and dead bacteria serving as cloud condensation nuclei (CCN). CCN are tiny particles that even smaller water vapor droplets cling to as raindrops form. Once enough water vapor droplets gather on the nuclei, raindrops fall. This is a critical part of Earth’s water cycle, which moves water from the oceans to the land, making freshwater and life on land possible. Without the nuclei, water vapor would not collect and fall as raindrops. Other types of condensation nuclei include mineral dust particles, salt from the ocean and sulfate from volcanic activity.

Seasons: Winter, Spring, Summer, Fall

Source: Smith, DJ et al. “The High Life: Transport of Microbes in the Atmosphere.” Eos 92 (2011): 249-250.

Forests react to climate change through changes in species composition. The biodiversity, or collection of different species with different strengths and vulnerabilities, of a forest enables it to adapt to changing conditions. For example, a drought tolerant tree species may only have a few members standing when rainfall is plentiful, while drought intolerant species with other positive characteristics are flourishing. The tables turn if a sustained period of drying causes increased mortality amongst the water loving trees, a process that will leave gaps in the canopy which the drought-tolerant species will fill. A survey of which species are present in mature form and which species are the most common saplings provides a good indication of where a forest has been and where it is going. Analysis of a heavily studied forest plot on Barro Colorado Island in Panama shows a forest with many drought tolerant species, which gained the upper hand during a dry period that ran from the 1950s until the mid-1980s and was capped-off by the severe 1983 El Niño related drought. Since the 1970s, the average temperature in the plot has warmed by 3.6 degrees Fahrenheit. This temperature rise has been accompanied by an increase in average annual rainfall and this increased moisture is accompanied by an increased abundance of drought-intolerant saplings.

Seasons: Winter, Spring, Summer, Fall

Source: Feeley, KJ et al. “Directional changes in the species composition of a tropical forest.” Ecology 92 (2011): 871-882.

Afternoon summertime precipitation in the wet, humid and heavily vegetated eastern United States is related to the evaporation that happens in the morning. Lots of soil moisture can stimulate lots of evaporation, which can push cloud development over the threshold where precipitation happens. This positive land-atmosphere feedback means that soil moisture conditions are self-reinforcing. Dry soils mean less evaporation and less plant-nurturing rainfall, while wet soils mean more evaporation that can enhance the probability of a rainfall event by up to 25 percent. Soil moisture conditions do not appear to be particularly influential regarding rainfall intensity, however, with other factors such as large-scale convergence largely controlling how much rain falls during a given summertime afternoon event. This coupling of processes at the land surface and the atmosphere helps to explain the persistence of droughts, as dry soil conditions make the atmosphere less conducive to drought-mitigating rainfall. It also explains the persistence of wet periods where ample soil moisture stimulates the precipitation that keeps it moist.

Seasons: Summer

Source: Findell, KL et al. “Probability of afternoon precipitation in eastern United States and Mexico enhanced by high evaporation.” Nature Geoscience 4 (2011): 434-439.

Earth’s weather is driven largely by the behavior of two large scale tropical circulation systems: the Hadley circulation and the Walker circulation. The Hadley circulation develops as large columns of warm, moist air rise over the equatorial regions, where the sun’s radiation is most intense (the “rising” regions are between 17.5 degrees North and South). These air columns then travel towards the poles, dropping rainfall along the way. Once this air has literally “run out of steam,” it becomes dry, sinking air that creates areas of divergence on the Earth’s surface, promoting deserts between the latitudes of 17.5 and about 40 degrees in the Northern Hemisphere and 17.5 and 35 degrees in the Southern Hemisphere (the “sinking” regions). Over the last 30 years, this cycle has intensified, with increasing precipitation over the rising regions and decreasing precipitation over the sinking regions. In other words, wet regions are getting wetter and dry regions drier. In the Northern Hemisphere during this 30 year period, the average boundary of the Hadley circulation ? defined as the point where the dry subsidence regions end and wetter climates begin – has been moving poleward in all seasons except spring. The trend is most pronounced during the summer months, where the boundary is now about six degrees farther north than it was in the late 1970s. Similar but weaker trends exist for the Southern Hemisphere, with only one to two degrees latitude shift during the summer and fall months. This trend suggests that Earth’s subtropical desert regions may be growing.

Seasons: Winter, Spring, Summer, Fall

Source: Zhou, ZP et al. “Recent trends of the tropical hydrologic cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data.” Journal of Geophysical Research: Atmospheres 116 (2011): D09101.

Earth’s weather is driven largely by the behavior of two large scale tropical circulation systems: the Hadley circulation and the Walker circulation. The Walker circulation can be broken down into three different systems? the Indian, Pacific and Atlantic Walker circulations ? each having a region of rising air and a region of sinking air. The best known Walker circulation is the system where warm, moist air rises over the western tropical Pacific Ocean between 120 and 180 degrees East (from the eastern tip of Borneo east to Fiji) and travels east, dropping rainfall along the way. By the time this air approaches the West Coast of South America, it descends as dry air to the surface. This sinking air creates a zone of divergence which keeps the weather off the West Coast of equatorial South America very dry and the skies clear. Clear skies allow lots of sunlight to hit the waters off Peru, which along with a frequent upwelling of cold, nutrient rich water, creates one of the richest fishing grounds on Earth. Variations in the strength of this Walker circulation drive the El Niño-Southern Oscillation, which influences weather around the world. The two other Walker circulations operate in a similar manner, with rising regions in the western tropical Indian and Atlantic Oceans, and sinking regions in the eastern tropical Indian and Atlantic Oceans. All of these circulations appear to have strengthened over the last 30 years. Increasing precipitation in the rising regions and decreasing precipitation in the sinking regions, means that the wet areas of the tropics are getting wetter and the dry areas drier.

Seasons: Winter, Spring, Summer, Fall

Source: Zhou, ZP et al. “Recent trends of the tropical hydrologic cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data.” Journal of Geophysical Research: Atmospheres 116 (2011): D09101.

Regional precipitation and temperature changes, atmospheric composition changes that affect plant growth, land use changes, changes in nitrogen deposition on the soil, and even changes in near-surface ozone concentrations, atmospheric aerosol loading and solar irradiance have all likely contributed to changes in outflow of the world’s major rivers over the past 60 years. There has been an overall decrease of global river flow since the late 1940s, with the world’s major rivers now sending about one less cubic kilometer of freshwater into the oceans each year. This is a rough figure, with a great variation of trends in the world’s rivers. The Amazon River, for example, is putting more water into the Atlantic Ocean every year, due primarily to the impact that higher atmospheric carbon dioxide (CO2) levels are having on plants in its basin. Higher atmospheric CO2 levels mean that less water exits the plants through their stomata, the tiny openings in their leaves that regulate the exchange of gases between the plants and the atmosphere. With less moisture going from the soil to the air through the plant leaves, more water goes into the streams and rivers that lead to the ocean. The Yangtze River in China, on the other hand, has less outflow due largely to land use changes dominated by more agriculture and irrigation. Despite land use, CO2 levels and changes in nitrogen (a plant fertilizer) deposition all playing roles, climatic changes are far and away the dominant factor in water cycle changes on both global and most regional scales. Compared to the late 1940s, the Mississippi River is sending an average two more cubic kilometers of water into the Gulf of Mexico each year, due partly to atmospheric CO2 increases but primarily to increased rainfall the central United States.

Seasons: Winter, Spring, Summer, Fall

Source: Shi, X et al. “The impact of climate, CO2, nitrogen deposition and land use change on simulated contemporary global river flow.” Geophysical Research Letters 38 (2011): L08704.

Boreal (Northern Hemisphere) spring is here. Even in the high northern latitudes, temperatures are beginning to warm and plants are beginning to come out of dormancy and photosynthesize, using the Sun’s energy to turn water and atmospheric carbon dioxide into sugar and grow. Snow cover may seem like a relic of winter, but snow also effectively reflects sunlight off the ground. In the boreal forests of the Northern Hemisphere, snow still covers much of the forest floor. Gaps in the tree canopy allow sunlight to hit snow on the ground, which reflects sunlight back towards the trees above. The amount of energy the trees receive from this reflection can even exceed the amount of energy they get from the initial downward wave of sunlight! It is likely that this reflection gives the trees a boost of energy, giving them the kick they need to “wake-up” and photosynthesize. This boost may be diminishing in importance as the duration of Northern Hemisphere snow cover has been shrinking at a rate of 5.5 days per decade over the past 40 years. Most of this decline in snow cover duration is due to melting in the late winter and early spring period.

Seasons: Spring

Sources: Choi, G et al. “Changing Northern Hemisphere Snow Season.” Journal of Climate 23 (2010): 5305-5310 and Pinty, B et al. “Snowy backgrounds enhance the absorption of visible light in forest canopies.” Geophysical Research Letters 38 (2011): L06404.

About 100 miles east of Salt Lake City, the Uinta Mountains of northeastern Utah rise up out of the desert. These mountains are the highest east-to-west mountain range in the contiguous United States.; most mountain ranges, such as the Rocky, Appalachian, Cascade and Sierra Nevada Mountain ranges all run from north-to-south. Today, there are no glaciers on the Uinta Mountains, despite peaks as high as 13,500 feet. 17,000 years ago, however, glaciers covered much of these ranges, with the feet of the glaciers being as low as 8500 feet. The dynamics of the glaciers at this time illustrate a few points about the most recent episode of deglaciation:
•    Despite the extent of the Laurentide, or North American, Ice Sheet reaching its maximum extent about 20,500 years ago, the glaciers in the Uinta Mountains did not reach their maximum extents until about 16,800 years ago. Glaciers in the Wind River range of Western Wyoming began their retreat at about the same time as the Laurentide Ice Sheet, while glaciers in southwestern Colorado did not start their retreat until 18,900 years ago and glaciers in north-central Colorado did not start to retreat until around 18.4 thousand years ago.
•    Glacier extents are affected by both temperature and moisture availability. As the North American ice sheet retreated, temperatures in northeastern Utah likely rose. At the same time , the jet stream moved further north and was frequently sitting over the Uinta Mountains, bringing in moist air masses that build glaciers.
•    To the west of the Uinta Mountains lie the remains of Lake Bonneville, once as large as Lake Michigan but today mostly desert with a few remnant water bodies such as the Great Salt Lake. Analysis of the ancient extents of the Uinta glaciers indicates that glaciers closer to the lake were larger than glaciers farther away. When Lake Bonneville started to shrink significantly around 17,000 years ago, the glaciers in the Uinta Mountains soon followed. This illustrates the importance of local moisture sources, such as large lakes, for local precipitation and glacial growth.

Seasons: Winter, Spring, Summer, Fall

Sources: Munroe, JS et al. “Latest Pleistocene advance of alpine glaciers in the southwestern Uinta Mountains, Utah, USA: Evidence for the influence of local moisture sources.” Geology 34 (2006): 841-844 and Dyke, AS et al. “The Laurentide and Innuitian ice sheets during the Last Glacial Maximum.” Quaternary Science Reviews 21 (2002): 9-31.

Image Left: Northern Hemisphere full snow season cover trends. Image Courtesy of the American Meteorological Society and may be used on-air and online provided proper attribution is given.

Source: Choi, G et al. “Changing Northern Hemisphere Snow Season.” Journal of Climate 23 (2010): 5305-5310.

Seasons: Fall, Winter, Spring

Source: Choi, G et al. “Changing Northern Hemisphere Snow Season.” Journal of Climate 23 (2010): 5305-5310.

In Brief: Temperatures in local waterways are rising from a combination of factors.

Moving from Chicago to Baltimore, where the average temperature is about ten degrees Fahrenheit warmer, might take a little bit of adjustment but almost certainly wouldn’t be fatal for humans. Most fish and other aquatic species, however, would not survive an equivalent temperature change. Warmer water temperatures have contributed to the elimination of the Brook Trout from many streams in the eastern U.S. In the mid-Atlantic region, temperatures are becoming intolerable for some sensitive species like the Longnose dace and Cutlips minnow. Temperatures are beginning to enter the danger zone even for the relatively tough and tolerant Blacknose dace, the most common species in the region’s urban streams.

Annual mean water temperatures in the nation’s streams and rivers are increasing at an average rate of 0.016 and 0.139 degrees Fahrenheit per year, respectively. The Potomac River around our Nation’s capital is seven degrees warmer than it was in the 1920s and the Delaware River around Philadelphia is 4.5 degrees warmer that it was in 1965. Some likely factors behind the rising water temperatures include:

• A Warming Climate: Warming water temperatures are linked to the rise in surface temperatures that have occurred over the same period.
• Land Use Changes: Surfaces like concrete, asphalt and rooftops hold more heat more than vegetated ground, making runoff from urban areas warmer than runoff from rural or forested areas. These hard surfaces also prevent water from soaking into the ground, leading to sudden discharges of warm waters into streams and rivers. Urban runoff during a summertime thunderstorm can raise a stream’s temperature by 12 degrees in less than 30 minutes.
• Loss of Trees: Fewer trees on stream banks mean that streams receive more direct sunlight, raising their temperatures.
• Thermal Power Plants: Increased demand for electricity has led to the construction of more thermal power plants over the last century, and these power plants discharge hot water.
• Dams: Dams create large bodies of standing water, which absorb more energy than running water.

Seasons: Spring, Summer, Fall

(Source: Kaushal, SS et al. “Rising stream and river temperatures in the United States.” Frontiers in Ecology and the Environment 2010; 100323112848094 DOI: 10.1890/090037.)

Trivia Question: Which of the following is a common source of cloud condensation nuclei?

a. Dust storms
b. Ocean salt spray
c. Volcanoes
d. Ocean algae
e. All of the above

The correct answer is e. Dust from dust storms, salt from the ocean, sulfate from volcanic activity, and a substance emitted in large quantities by ocean algae blooms called dimethylsulfide are all crucial sources of cloud condensation nuclei. Any changes in the concentrations of these different nuclei can affect the weather by affecting when and how clouds form and rain falls.

Seasons: Winter, Spring, Summer, Fall

Source: Vakkubam SM et al. “Weak response of oceanic dimethylsulfide to upper mixing shoaling induced by global warming.” PNAS 104 (2007): 16004-16009.

Trivia Question: As temperatures have warmed over the past 40 years, the area of the Green Mountains dominated by pines has:

a. shrunk
b. expanded
c. remained about the same

The correct answer is a. A two degree Fahrenheit warming and a 40 percent increase in precipitation in the region over the past 40 years has corresponded to shrinking of the area dominated by conifer trees and range expansion of the less cold hardy broadleaf trees. The area of the mountains dominated by broadleaf forests increased by 19 percent.

Seasons: Winter, Spring, Summer, Fall

Source: Beckage, B. et al. “A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont.” PNAS 105 (2008): 4197-4202.

Trivia Question: Have such prolonged dry episodes become more or less common over the past 60 years?

a)    More common
b)    Less common
c)    No change

The correct answer is b. Despite drought conditions in the late 1990′s and early 21st century, there appears to be an overall trend of fewer prolonged dry events in the Southwest since the 1950′s. This is especially true for the cold season (October through March), due to the more El Niño events in the eastern tropical Pacific Ocean over the study period. El Niño events work to steer the Northern Hemisphere storm track right over the desert Southwest. Also, since the mid-1970′s, the North Pacific Ocean has been in a “warm” phase of the Pacific Decadal Oscillation. Warm phases help to enhance the effects of El Niño events on Southwest rainfall. An overall warming, however, and thus an increase in soil evaporation, may be counteracting the effect this increase in precipitation has on streamflow levels.

Seasons: Winter, Spring, Summer, Fall

Source: McCabe, GJ et al. “Variability and trends in dry day frequency and dry event length in the southwestern United States.” Journal of Geophysical Research: Atmospheres 115 (2010): D07108.