Multiple observation sources document widespread warming of the Earth in the last century. Although this trend is evident global, year-to-year variability makes detecting this trend problematic and interpreting climate records at fine spatial and temporal scales should be done very carefully. The Intergovernmental Panel on Climate Change (IPCC) recently concluded that warming of the climate was unequivocal and that most of the observed changes in global average temperature since the mid-20th century were very likely due to observed increases in anthropogenic greenhouse gases. The human influence on climate is even emerging from the noise of year-to-year variation at the scale of western North America. Carbon dioxide (CO2) from the burning of fossil fuels is the largest anthropogenic source of greenhouse gas emissions. Carbon dioxide has risen rapidly in the last 50 years, transforming the Earth’s natural carbon cycle.
In the Northwest, we have had observed changes of temperatures of about 1.5 ºF in the last century, and this change in temperature is evident in hydrologic shifts. We have seen observed decreases in snowfall events in the last 50 years and declining April snowpack. Streamflows are also changing as winter flows rise and summer flows drop. Future climate for the Northwestern United States. is likely to see increases in temperature of roughly 0.5 ºF / decade. Future precipitation is fairly uncertain, although several models have winter precipitation increasing and summer precipitation decreasing. Present-day patterns of greenhouse gas emissions constrain the rate of change of temperature for the next few decades: we are committed to some degree of additional climate change. Beyond mid-century, the projections of warming depend increasingly on emissions in the next few decades and hence on actions that would limit or increase emissions.