Among the effects documented are a step-like increase of nearly 2°C in air temperature (S.A. Bowling, Geophysical Institute, University of Alaska Fairbanks, personal communication, 1995), an approximate 5% reduction in sea-ice extent (Niebauer 1998), and a decrease in sea-ice thickness (Wadhams 1995). Many local residents around the Bering Sea also noted changes in ice thickness and strength (Huntington 2000). Permafrost temperatures measured in boreholes in northern Alaska are 2-4°C warmer than they were 50-100 years ago (Lachenbruch and Marshall 1986). Discontinuous permafrost (i.e., permafrost that is patchily distributed over the landscape) has warmed considerably and is thawing in some locations (Osterkamp 1994). In addition to the warming trend of air temperatures, marked changes have occurred in atmospheric pressure patterns, circulation, cloudiness, precipitation, and evaporation. Some North American regions are experiencing an increase in runoff (due to increased rain) of major rivers and changes in the time of river-ice breakup and the onset of the summer peak in river flow. In addition, south coastal Alaska glaciers have decreased because of melting, which has increased freshwater discharge rates nearly 15% (Arendt et al. 2002, Royer in press). Multiple air temperature signals exist in the climate record. One signal is a trend to warmer temperatures in recent decades, while many of the other natural patterns have alternating warm/cold periods, such as Arctic Oscillation (AO) and El Niño-Southern Oscillation (ENSO) (NRC 2001, 2003). The environment of AYK salmon is changing, possibly due to warming and associated climate variations that are occurring throughout the Bering Sea and Alaska.
This section emphasizes seasonal and longer fluctuations in the air, land, and sea environments. However, we recognize that episodic events can also influence salmon populations. Floods, as extreme hydrological events, can affect water quality and may scour gravels and deposit fine-grained sediment, thereby damaging spawning beds and/or flushing young fish out of the river (Brabets et al. 2000). Three major floods have occurred in the Yukon River basin since 1949 (Brabers et al. 2000): in 1964 (June/July, due to melt of large snow pack), in 1967 (12-18 August, in the middle and lower Tanana River basin with a magnitude estimated to be twice the 100-year flood discharge), and in 1994 (15-27 August, in the upper Koyukuk River basin). At the other extreme are droughts, which can inhibit upstream migration as well as affect eggs after spawning through increased water temperature, decreased concentration of dissolved oxygen, and even dewatering of the redds. Extended droughts also influence groundwater levels that, in turn, decrease the base flow of