In the past, climate variations were caused by natural forces. These include external changes in amount of energy the Earth receives from the Sun, or the cooling effects of dust clouds from powerful volcanic eruptions, amplified by natural feedbacks within the earth-ocean-atmosphere system. Today, however, the climate is being altered by both natural and human causes (Hegerl et al., 2007). Since the Industrial Revolution, atmospheric levels of heat-trapping gases such as carbon dioxide and methane have been rising, primarily due to increased consumption of fossil fuels such as coal, oil, and natural gas (Andres et al., 1999; Stern & Kaufmann, 1998).
Atmospheric levels of carbon dioxide are now higher than they have been at any time in at least the last 800,000 years (Lüthi et al., 2008). Average surface temperatures in the Northern Hemisphere have risen by 1.3°F (0.75oC) over the past 150 years (Trenberth et al., 2007). Based on these and many other lines of evidence, the Intergovernmental Panel on Climate Change (IPCC), which represents the work of thousands of climate scientists around the world, has concluded that it is very likely that most of the climate changes observed over the last fifty years have been caused by emissions of heat-trapping gases from human activities (IPCC, 2007). Subsequent analyses have strengthened this conclusion, with more recent studies suggesting that human influence is responsible for most of the warming over the last one hundred and fifty years, and as much as all of the warming over the last sixty years (Huber & Knutti, 2011; Foster & Rahmstorf, 2011; Gillett et al., 2012).
Over the coming century, climate will likely continue to change in response to both past and future emissions of heat-trapping gases from human activities (IPCC, 2007). At the global scale, temperature increases between 4oF up to 13oF are expected by end of century, accompanied in many regions of the United States by increases in extreme heat and heavy precipitation events (USGCRP, 2009).
Alabama's climate - together with that of the rest of the United States - is expected to reflect changes occurring at the global scale (USGCRP, 2009). This report describes the changes in long-term climate and climate variability that might be expected over the coming century for five long-term Global Historical Climatology Network (GHCN) weather stations in the Mobile Bay region of Alabama, located in Bay-Minette, Coden, Fairhope, Mobile (airport), and Robertsdale. Future projections are based on three future emissions scenarios (higher, mid-high, and lower), and simulations from ten different global climate models.
Section 2 lays out a general framework for conducting regional climate impact analyses, describes the scenarios and models, and explains the statistical downscaling model used to generate high-resolution projections for the individual weather stations.
Section 3 discusses how infrastructure-relevant temperature metrics are likely to be affected by climate change in the near future (2010-2039), by mid-century (2040-2069) and towards the end of the century (2070-2099) relative to a historical baseline of 1980-2009.
Section 4 describes projected changes in precipitation indices for those same future time periods.
Section 5 provides guidance on understanding and interpreting the range of uncertainty in future projections, and evaluates the ability of downscaled climate projections to reproduce observed historical variability for the Mobile stations.
Finally, Section 6 concludes with a discussion of the implications of climate change for the Mobile Bay region, including the potential for climate projections to inform adaptation planning.