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PUBLISHED: 5:19 PM on Wednesday, September 10, 2008
Century's forecast: Warmer and wetter
JUNEAU - The more distant the meteorological forecast, the hazier the prediction. Still, meteorologists are reasonably confident in this century's outlook for Southeast Alaska.

"It is fairly well accepted that our climate is going to be warmer and wetter," said Tom Ainsworth, meteorologist in charge at Juneau weather forecast office, part of NOAA's national weather service, which forecasts the weather for all of Southeast.


  Cover illustration by Joel Irwin
Atmospheric temperatures worldwide are rising, and Southeast Alaska is no exception. In a 2007 report to the mayor and assembly of Juneau, a team of scientists, including Ainsworth, looked at observed changes in climate and predictions for the coming century. But how much warmer and wetter will it get?

In the last 60 years, Juneau's average winter temperature has risen at least 1.5 degrees and possibly as much as 3 degrees, according to the report. It is expected to rise an additional 10 degrees by the end of the 21st century.

At sea level, this temperature increase will result in more precipitation falling as rain instead of snow. Average winter snowfall at sea level decreased from 109 to 93 inches in the last 60 years. In that same time, precipitation levels have risen by at least 2.6 inches. Precipitation levels are determining by melting snow; on average, for every 10 inches of snow that falls, one inch of precipitation is recorded. So, more precipitation falling as rain results in more precipitation recorded overall.

Land and sea

Climate change is reflecting itself in Southeast Alaska in ways both obvious and counterintuitive. Glacier melting is one of the most well known indicators of warming climates. In Juneau, the Mendenhall Glacier has been retreating for centuries but the current rate is so striking that repeat summer visitors would notice a difference from one year to the next. But while ice fields melt, yellow cedar trees are freezing due to the loss of insulating snow cover in spring.

As glacial ice melts and ocean waters expand as they warm, global sea levels are expected to rise over the next century, causing concern for coastal settlements around the world. But Southeast Alaska will in fact see a decrease in relative sea level, according to the report. Earth formerly under the pressure of a glacier will rise when that glacier retreats. The process, called isostatic rebound, is resulting in an uplift of land greater than the sea level rise.

Climate vs. weather

One of the trickiest things to make sense of in terms of climate change is the same thing that is easiest for everyone to see: the daily weather.

The study of weather and the study of climate are intertwined: Weather patterns over time determine the climate, and the climate is in turn used to predict weather patterns.

"Climate is the statistics of weather," explains Rick Fritsch, a Juneau forecaster who specializes in climate. Or there's the Mark Twain definition: "Climate is what we expect, weather is what we get."

Looking at an area's past weather conditions gives meteorologists an idea of what weather conditions are plausible for a given area during a given time of year. Forecasters can feel more confident with a forecast if it is within a range of probable outcomes.

"With climate we're trying to establish some baseline statistics, what constitutes an extreme event," Ainsworth said.

The more data meteorologists have to draw on, the more accurate their understanding of what is probable and not. Juneau, like much of the western United States, does not have a very extensive climate record to draw on. Whereas meteorologists on the East Coast could have hundreds of years of records, Juneau meteorologists only have records dating from the 1880's onward. These records are inconsistent before 1943, when an official weather recording station was first installed at the Juneau airport.

A quivering base

Only by looking at these baseline statistics, accumulated over a long period of time, can climate change in a region be observed. Very little can be inferred based on weather during a short period of time.

"You can't really gauge climate change by what's going on outside today, or even this year," Fritsch said. "I can't really look to the last three years or the last five years and see any signal there."

Southeast Alaskans may not feel any evidence of global warming after a summer as cold and wet as this one, but this summer's weather can almost be chalked up to luck of the draw. Ainsworth compares a summer forecast to a roulette wheel: Either the ball falls in the black slot or the red; either the summer is cool and rainy or warm and sunny.

"Just because we had a cold, wet summer doesn't mean global warming is a bunch of hot air," Ainsworth said.

Most meteorologists are certain that climate change is occurring, and that it is driven in part by human activity. However, they are still learning how to reconcile the changing climate with day-to-day predictions. The science of weather prediction must take into account the norm, and now that norm is in the process of changing.

"Climatology is our base," Ainsworth said, "and right now our base is sort of quivering."

With a quivering base, meteorologists themselves can be surprised by the weather. For example, in the winter of 2006-2007, Juneau meteorologists predicted above-average snowfall, but did not expect close to the 200 inches Juneau received, Ainsworth said.

In the coming years, meteorologists in Juneau and around the world will need to figure out how to best to incorporate climate change into short-term weather predictions. While Juneau meteorologists are fairly certain that the coming years will bring warmer and wetter winters, they are still learning how to best use that knowledge to predict weather in the near future.

"It's still pretty uncertain territory," Ainsworth said. "It's still a pretty fresh area of study. It's not clear cut."

The report prepared in April 2007 on Climate Change in Juneau can be viewed online at http://www.juneau.org/clerk/boards/Climate_Change/CBJ%20_Climate_Report_Final.pdf.


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