The Earth’s climate is driven by the sun. The overall process is best understood by considering the Earth’s energy budget (see sidebar). Each of these processes influences climate through changes to temperature, winds, the water cycle, and more. Incoming solar radiation may be reflected, absorbed by land surface and water bodies, transformed (as in photosynthesis), or emitted from the land surface as longwave radiation. NATURAL AND HUMAN CAUSES OF CLIMATE CHANGEĬlimate is driven largely by radiation from the sun. In the sections below, we introduce the climate science and discuss important fundamental processes that determine whether climate remains constant or changes. This foundation then serves as the basis for analyzing three key sectors of Montana-water, forests, and agriculture-considered in the other chapters of this assessment. This chapter serves as a foundation for the Montana Climate Assessment, providing information on present-day climate and climate terminology, past climate trends, and future climate projections. considering what today’s best available climate models project regarding Montana’s future, and how certain we can be in those projections. reviewing historical trends in Montana’s climate, and what those trends reveal about how our climate has changed in the past century, changes that are potentially attributable to world-wide increases in greenhouse gases and.providing a baseline summary of climate and climate change for Montana-with a focus on changes in temperature, precipitation, and extreme events-including reviewing the fundamentals of climate change science.Climate change encompasses both increases and decreases in temperature, as well as shifts in precipitation, changing risk of certain types of severe weather events, and changes to other features of the climate system.” The US Global Change Research Program (USGCRP undated) defines climate change as follows: “Changes in average weather conditions that persist over multiple decades or longer. The largest decreases are expected to occur during summer in the central and southern parts of the state. The largest increases are expected to occur during spring in the southern part of the state. Īcross the state, precipitation is projected to increase in winter, spring, and fall precipitation is projected to decrease in summer. Increases in the number of frost-free days are expected to be greatest in the western part of the state. Increases in the number of days above 90☏ (32☌) are expected to be greatest in the eastern part of the state. The number of days in a year when daily temperature exceeds 90☏ (32☌) and the number of frost-free days are expected to increase across the state and in both emission scenarios studied. These state-level changes are larger than the average changes projected globally and nationally. By the end-of-century, Montana temperatures are projected to increase 5.6-9.8☏ (3.1-5.4☌) depending on the emission scenario. By mid century, Montana temperatures are projected to increase by approximately 4.5-6.0☏ (2.5-3.3☌) depending on the emission scenario. The state of Montana is projected to continue to warm in all geographic locations, seasons, and under all emission scenarios throughout the 21 st century. A significant increase in spring precipitation (1.3-2.0 inches ) has also occurred during this period for the eastern portion of the state. Average winter precipitation has decreased by 0.9 inches (2.3 cm), which can mostly be attributed to natural variability and an increase in El Niño events, especially in the western and central parts of the state. ĭespite no historical changes in average annual precipitation between 19, there have been changes in average seasonal precipitation over the same period. In addition, the annual number of warm days has increased by 2.0% and the annual number of cool nights has decreased by 4.6% over this period. From 1951-2010, the growing season increased by 12 days. Montana’s growing season length is increasing due to the earlier onset of spring and more extended summers we are also experiencing more warm days and fewer cool nights. Average temperatures during these seasons have risen by 3.9☏ (2.2☌) between 19. Winter and spring in Montana have experienced the most warming. The increases range between 2.0-3.0☏ (1.1-1.7☌) during this period. Annual average temperatures, including daily minimums, maximums, and averages, have risen across the state between 19.
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