20^th CENTURY CLIMATE TRENDS IN BRITISH COLUMBIA

          British Columbia (49°-60° latitude, 948,000 square kilometers) is a large province that encompasses a broad diversity of ecosystems (Figure 2).  The diversity of the province is due in large part to the extraordinary heterogeneity of landscapes and ecological zones, which encompass everything from temperate rainforest coastal areas to savannah interior valleys to arctic mountain ranges.  Climate in British Columbia is extremely variable due to these topographic, latitudinal and continental complexities.  For examining climate trends, BC will be referred to in terms of climatic and forest trends as related to ten climatically, topographically and geologically distinct ecoprovinces (Ministry of Water, Land and Air Protection 2002).

Figure 1: British Columbia’s Biogeoclimatic Ecological Classification zones.

In addition to biogeographic reasons for climatic differences throughout the province, climate in British Columbia oscillates on a yearly and decadal scale due to Pacific air currents.  An important challenge in quantifying and projecting climate change is teasing apart these natural fluctuations from anthropogenically-induced climate forcing due to greenhouse gas buildup. 

The two most important cyclic climate influences in BC are the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) (Ministry of Water, Land and Air Protection 2002).  Both phenomena are associated with cyclic changes in the surface temperature of the Pacific Ocean that impact air temperature and precipitation throughout the Pacific.  ENSO events tend to develop in the spring and last from 6 to 18 months.  In southern British Columbia, ENSO events are typically associated with warmer, drier winters and reduced snow pack (Shabbar and Bonsal 1997).  La Niña, the cool phase of ENSO, is usually characterized by negative temperature and positive precipitation anomalies.  PDO phases, on the other hand, are much longer, usually in the realm of 20-30 years.  Climate records show that a PDO “cool” regime dominated from 1890-1924 and from 1947-1976, while a “warm” regime prevailed from 1925-1946 and from 1977 through at least the mid-1990s (Figure 2) (Mantua and Hare 1997).  Precipitation and temperature extremes are exacerbated during years in which PDO and ENSO are in the same phase.

Figure 2: PDO fluctuations over the 20^th century.

          Anthropogenic climate forcing can be extracted from the influence of PDO and ENSO through an examination of climatic trends influenced by more than a full cycle of PDO.  This entails looking at climate data on a scale greater than 60 years.  This sort of analysis not always possible due to lack of historical weather station records; however, extensive research has gone into recreation of pre-20^th century climate records from tree ring chronologies (Bunn and Graumlich 2005, Mann et al. 1999) and ice cores. Unless stated otherwise, the climate data reported below is based solely on trends significant at the 95% level for weather station climate records.  This means that in general trends from the southern half of the province are based on approximately 100 years of data while those from the north are based on approximately 50 years of data.  Correspondingly, the northern climate trends are more likely to show strong PDO influence while those from the south are considered to be largely independent of annual and decadal oscillations.  “No trend” indicates long (>60 years) data records that fail to show significant climate trends.

          Coastal BC has warmed 0.6°C, interior BC by 1.1°C and northwestern BC by 1.7°C since the late 1900s (Ministry of Water, Land and Air Protection 2002) (Figure 3).  No trend was found for the northeastern part of the province. 

Figure 3: Change in annual temperature, 1895-1995 (°C per century) (Ministry of Water, Land and Air Protection 2002).

Most of the warming has occurred in two distinct time periods: 1910 to 1945 and 1976 to 2000.  While these time periods are highly correlated to warm phases of the PDO, the warming trends are unprecedented for the last millennium.  The 1990s were the warmest decade in the last 1,000 years, based on a variety of tree ring and ice core chronologies (Ministry of Water, Land and Air Protection 2002).

Seasonal breakdowns of the warming trends reveal a strong increase in spring (March to May) temperatures throughout the province, summer (June to August) warming in the interior and winter (December to February) warming in the Coast and Mountains Ecoprovince and in the sub-boreal interior (Figure 4). 

Figure 4: Change in seasonal temperature, 1895-1995 (°C per century) (Ministry of Water, Land and Air Protection 2002).

The Southern Interior ecoprovince experienced notable warming during all four seasons.  An analysis of maximum and minimum temperatures shows a near-ubiquitous trend towards warmer nighttime temperatures across the province, particularly in spring and summer (Figure 5).  Daytime maximums increased in the Southern Interior and Southern Interior Mountains ecoprovinces, and springtime maximums increased by over 1°C throughout the interior. 

          One could say that British Columbia is not so much getting warmer as becoming “less cold”, as reflected by the upward trend in year-round minimum and spring maximum temperature highs.  The implications of these trends are profound in regards to the growth and survival of species in forest ecosystems.  Specific physiological impacts to trees of these climate trends include timing of spring budburst, growing season duration and pathogen survivorship as dictated by winter minimum temperatures.

Figures 5: Changes in maximum (left) and minimum (right) temperatures by season, 1895-1995 (°C per century) (Ministry of Water, Land and Air Protection 2002).

          Precipitation trends are available for the southern part of the province and show increases of 2% per decade in the Coast and Mountains and Central Interior ecoprovinces, 3% per decade in the Southern Interior and 4% per decade in the Southern Interior Mountains (Figure 6).  Projections are that precipitation has increased by almost 2% per decade in the northern part of the province as well.  Seasonal trends indicate a bias towards increasing spring precipitation in the central and southern interior and summer precipitation in the Southern Interior and Southern Interior Mountains (Figure 6).

Figure 6: Changes in annual (left) and seasonal (right) average precipitation, 1929 -1998 (% per decade) (Ministry of Water, Land and Air Protection 2002).

          Precipitation tends to be more locally variable than temperature, in part due to the impact of topography on weather fronts.  Increased precipitation could mean more soil moisture availability in forest ecosystems.  However, coupled with climatic trends towards greater year-to-year variability and extreme precipitation events, this might not necessarily translate to enhanced growth and survivorship conditions for trees.