Climate, Water and the Southern Salish Sea Islands

By Dan Moore, PhD

Climatic conditions have varied throughout earth’s history. About 17,000 years ago, the Salish Sea was covered by glacial ice as a result of globally cooler conditions.1 In addition to the longer-scale processes associated with past glacial cycles, the climate system includes shorter-time-scale phenomena such as El Niño events (in which a band of warmer-than-average water in the equatorial Pacific develops about every two to seven years2), which are usually associated with warmer-than-average winter conditions in south coastal BC3. Beginning in the 19th century, a long-term global warming trend has been added to the other types of climatic cycles and variability, associated with increases in greenhouse gases, especially carbon dioxide, which absorb heat emitted by the earth’s surface and re-emit it back to the surface. 

The southern Salish Sea Islands (here referring to the San Juan and southern Gulf Islands) experience a mediterranean climate, with mild, wet winters and warm, dry summers. Soils remain near “field capacity” (saturated) through winter, then dry out through spring, summer and early fall. The return of the wet season, typically in October, recharges soil moisture, bringing it back up to field capacity in November. 

Seasonal variations in precipitation and soil moisture influence both surface water and groundwater. When soils are wet in winter, almost all of the rain reaching the surface either drains downslope through or over the soil toward surface water bodies (lakes, ponds, wetlands and streams), or percolates deeply to recharge groundwater. When soils are dry, particularly in late summer, any rain that falls and percolates into the soil is retained in the soil, where it can be lost by evaporation from the soil surface or taken up by plants through their roots and released to the atmosphere through openings on their leaves, a process called “transpiration.” Through the spring and summer, the flow in many streams drops to a trickle or stops altogether, and water levels in ponds, lakes and wetlands decline. With reduced input via deep percolation from the soil, groundwater levels drop through the summer as aquifers continue to discharge directly into the Salish Sea or into surface water bodies. Groundwater use by pumping from wells during summer exacerbates this natural seasonal decline.

The longest available weather record in the southern Salish Sea islands is from the “Olga 2 SE” station on Orcas Island in Washington State. The general long-term trends at that station should be reasonably representative of those experienced throughout the San Juan and southern Gulf Islands. The graphs in Figure 1 show air temperature (T) and precipitation (P) as recorded at Olga 2 SE for winter (November to April) and summer (May to September).4  

Figure 1: Time series of total precipitation and average maximum daily air temperature at the Olga 2 SE weather station on Orcas Island for winter (November to April) and summer (May to September). The vertical dashed lines indicate the timing of shifts in the Pacific Decadal Oscillation.

The grey lines in Figure 1 illustrate the substantial year-to-year variability of climatic conditions, while the black lines indicate smoothed trends. The smoothed trends include the effect of the Pacific Decadal Oscillation, or PDO, which involved large-scale changes in sea surface temperatures and wind patterns in the North Pacific Ocean around the years 1922, 1947 and 1977.5  In southwestern BC, there was a tendency to cooler, wetter weather from the late 1940s to the late 1970s.  One notable feature of the graphs is the trend to warmer summers beginning in about the 1970s.

To understand how climatic conditions may change in the future, research teams around the world run global climate models. Projections of future conditions based on these models are subject to uncertainties because all models have inherent limitations, and it is uncertain how future human activity will influence the emission of greenhouse gases. Scientists and planners therefore consider outputs from a range of models run with a range of future scenarios about human activity.

A report published by the Capital Regional District presented a synthesis of climate scenarios for the region.6  Table 1 summarizes the projected changes in total seasonal precipitation and average daily maximum air temperature for the 2050s under a “business-as-usual” emissions scenario. The changes shown are relative to conditions in the period 1971-2000, and the ranges represent uncertainty associated with the variability among climate models as well as natural climatic variability.

SeasonPrecipitation change (%)Daily maximum air temperature change (°C)
Winter (DJF)-3 to 111.3 to 3.3
Spring (MAM)-5 to 141.6 to 4.5
Summer (JJA)-41 to 42.1 to 4.2
Autumn (SON)-4 to 261.4 to 3.8

Table 1. Summary of projected climate changes for the 2050s, relative to conditions in 1971-2000, for the Capital Regional District.

There is a strong consensus among models that air temperatures will increase in all seasons and that summers will likely be drier and autumns wetter. Changes in overall precipitation in winter and spring are less clear. These projected climatic changes could have a range of impacts on water and associated values in the Salish Sea Islands. 

Groundwater provides the dominant source of water for domestic, agricultural and industrial uses on most Salish Sea islands. A study of projected groundwater response to future climate change on Gabriola Island found that there would be an overall increase in annual precipitation and an associated increase in groundwater recharge by the 2050s.7 However, the increase in recharge did not necessarily translate into higher groundwater levels, because much of the increased winter recharge was balanced by increased discharge from the aquifer.

Increasing air temperatures and reduced precipitation during the growing season would increase the need for irrigation for agriculture and landscaping. Where groundwater is used for irrigation, this increased demand would put more pressure on groundwater resources. Where surface water is used (e.g, in the form of storage ponds), increasing air temperature would likely result in increased evaporation losses at the same time as reduced precipitation would reduce inflows.  

Warmer, drier summers would create higher drought stress, with implications for forest health. For example, since the 1990s, western red cedar have been exhibiting signs of drought stress, and many trees have died, as reported by CBC News (May 14, 2019).8  The warming trend could also result in higher forest fire hazard that begins earlier in the spring and extends later into the fall. An increase in standing dead trees could exacerbate fire hazard by adding to the fuel load. Available evidence from historical climatic data and projections from global climate models indicate that the southern Salish Sea Islands will likely experience warming in all seasons and reduced summer rainfall, with implications for water availability and ecosystems. The essays in the rest of this special issue of Stewardship News explore the connections between water conservation and climate, as well as a range of innovative solutions to current and future challenges.


1 John J. Clague and Brent Ward, “Pleistocene glaciation of British Columbia.” In J. Ehlers, P.L. Gibbard and P.D. Hughes, editors: Developments in Quaternary Science , Vol. 15, Amsterdam, The Netherlands, pp. 563-573, 2011.


3 R.D. Moore, D.L. Spittlehouse, P.H. Whitfield, P.H. and K. Stahl, “Weather and climate.” Chapter 3 in: R.G. Pike, T.E. Redding, R.D. Moore, R.D. Winkler and K.D. Bladon (editors). Compendium of forest hydrology and geomorphology in British Columbia . B.C. Ministry of Forest and Range, Forest Science Program, Victoria, B.C. and FORREX Forum for Research and Extension in Natural Resources, Kamloops, B.C. Land Management Handbook 66, pp. 47-84, 2010.

4 Data are available via the following link:

5 See P.H. Whitfield, R.D. Moore, S.W. Fleming, and A. Zawadski, “Pacific Decadal Oscillation and the hydroclimatology of Western Canada – review and prospects.” Canadian Water Resources Journal Vol. 35, pp. 1-28, 2010.

6 Capital Regional District, “Climate Projections for the Capital Region.” Victoria, 59 pp., 2017.

7R. Burgess, “Characterizing recharge to fractured bedrock in a temperate climate.” MSc thesis, Simon Fraser University, Burnaby, Canada, 2017.