By Amy Grotta, OSU Forestry & Natural Resources Extension Agent – Columbia, Washington & Yamhill Counties

In this series, our goal is to discuss how woodland owners and managers might want to think about the management decisions we make in light of anticipated climate change. To do that, we need to understand what’s potentially in store. What are the future climate projections for our region, and how do they differ from what we are accustomed to? What is the relationship between climate and weather? That’s what this article aims to address. Future articles will dive into how these changes might affect our forests, and how we can respond.

Weather stations like this Snotel station provide long-term climate data for many locations across the landscape. Photo: NRCS Oregon
Weather stations like this Snotel station provide long-term climate data for many locations across the landscape. Photo: NRCS Oregon

Let’s start with Oregon’s climate as we know it. This may seem obvious, but it bears summarizing, for comparison to future scenarios. The Oregon Climate Service does a good job describing Oregon’s diverse climate for the uninitiated. Our varied geography is responsible for the climatic diversity we see across the state: how much precipitation falls, and whether it falls as rain or snow; how hot it gets during the day and how cold it gets at night. These patterns show up when we look at long-term weather data for a given site. Because the weather is so varied from year to year, and even from one decade to another, climatologists use observations from a 30-year period to describe average conditions on a site. These are called 30-year normals. The most recent normal data available are from the 30-year period from 1981-2010.

Clatskanie 1981-2010          Enterprise 1981-2010

The graphs show 30-year average climate data for two sites in Oregon (Clatskanie and Enterprise). By reducing the year-to-year variation to averages, climate patterns for a site become clear. For example, Enterprise tends to have much wider temperature swings each day than Clatskanie. Typical of western Oregon, Clatskanie gets most of its precipitation in the winter, whereas it is more constant across the year – and much lower overall – in Enterprise. Data like this is available from the Oregon Climate Service for dozens of weather stations across the state, and for different 30-year time periods.

These expected climate patterns guide many of the decisions we make as forest managers, and they have shaped many of the policies and best management practices related to forestry. Some examples:

  • how far seedlings can safely be moved from their seed source without risk of maladaptation;
  • what time of year is considered optimal to plant seedlings;
  • what time of year stream flows are at their lowest, to allow instream work;
  • what time of year fire season is expected to start and end.

Put simply, the relationship between weather and climate is a matter of scale: weather describes the conditions of the atmosphere over a short period of time, and climate is the expected atmospheric behavior over long periods of time (more here). So climate change is the change in long-term averages of daily weather.

The 2014 National Climate Assessment, produced by a team of hundreds of scientists, summarizes the best available science on past climate and projected climate change in the U.S. Findings are broken down regionally. For the Northwest (Oregon, Washington and Idaho), the NCA tells us:

  • It’s been gradually getting warmer over the last century, and that trend will continue. Of course, there will continue to be cold years and warm years; but overall, the trend is warmer – anywhere from 3 to 10 degrees warmer, on average, by the end of the 21st century. The largest temperature increases will be felt in the summer.
  • Precipitation will continue to be highly variable from year to year. That is, any long-term change in annual or seasonal precipitation is not expected to be noticeable. However, due to the warming trend, less of the precipitation will be snow, and more will fall as rain.
Trees along the Willamette Valley margins showed drought stress early in 2015.
Trees along the Willamette Valley margins showed drought stress early in 2015.

The last few years, we’ve experienced weather that is indicative of what the future may look like. For the winter 2014-15, while total precipitation was close to normal, we had record low snowpack and by May all of Oregon was experiencing some level of drought.  June 2015 was the warmest June on record in many parts of the state. We can’t attribute a single hot year to climate change; just as we can’t dismiss climate change when the snow piles up (like it did in February 2014; despite the fact that 2014 was Oregon’s warmest year on record). But, where these warm years may have seemed like an exception in the past, in the future they may seem more ordinary.

The next articles in this series will examine some of the nuances in interpreting climate science, and look at how trees and forest ecosystems may respond to these changes.



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