Since early July 2017 a series of intense wildfires have been recorded over Canada, caused chiefly by lightning strikes and driven by winds and dry surface conditions. Although wildfires are a common feature within the boreal forests of Canada during the summertime, the recent fires in British Columbia have created a plume of smoke and trace gases that appears to rank amongst the biggest in the satellite era.
Wildfires affect many areas of the world and alongside the local hazards such as loss of life and damage to property, the largest fires can send aerosol and trace gases high in to the atmosphere which can then readily be transported across the globe in a matter of days to weeks. In some cases smoke from intense fires can be injected directly into the stratosphere through pyrocumulus clouds and the consequences of these smoke particles on the radiation budget and stratospheric chemistry are still poorly understood.
At the beginning of July 2017, Canada has experienced a quiet start to the wildfire season with only around 50% of the 10-year average activity and less than 20% of average burned area and only three fires had been recorded in British Columbia, having burnt 147 ha. Over the course of the next two weeks these fires spread and an extra 24 fires were recorded and the area burnt increased rapidly to over 200000 hectares. With no relief from rains, by mid-August over 1000 separate fires had been recorded and the burnt area grew to almost 900000 ha (around 9000 square km). This figure is record-breaking for British Columbia and exceed the 8550 square kilometres burnt during the previous record set in 1958.
Satellite datasets offer the perfect platform with which to observe wildfire events such as these, which cannot easily or practically be measured by in-situ data and provides timely results. Imagers such as VIIRS on Suomi-NPP or OLCI on Sentinel-3 are two such instruments which provide a daily, near-global coverage, and provide information on aerosols and clouds with visible channels which can easily distinguish aerosols. On the 16th August, VIIRS measured a plume of smoke which had been transported from the mid-latitudes of British Columbia to the Arctic Circle and over Greenland.
Although circulation patterns have so far prevented direct transport of the smoke aerosol and gases to the ice mass of the Arctic there has been significant transport over the icy regions to the North of Canada and Greenland. If darker smoke particles deposit on snow or ice, can directly decrease the albedo of the surface thereby increasing the amount of solar radiation absorbed at the surface and in turn melting the snow layer. On 17-18th August, OLCI observed a large area of dark smoke over Baffin Bay, Figure 2. It is not yet known what the fate of these particles was.
At times over 400 firefighters were working to contain the flames but on 21st August it was reported that 19 of the fires had merged into a single fire covering and area estimated to be 4,674 square kilometres in size (3 times as large as London). This system released huge amounts of energy driving a number of pyrocumulus clouds which can readily transport pollutants into the upper troposphere. Late on 19th August, the first clear evidence of pollution transport across the Atlantic Ocean was observed by the SEVIRI instrument on MSG-3. Figure 3 shows the smoke plume on the afternoon of 19th August heading towards western France which was picked up by a number of lidar instruments and reported at heights of between 10 and 15 km. Due to the high altitude and small size of the particles, a visual effect that many people observed from the ground was a redder than usual sunset due to the increased scattering caused by these smoke particles.
The smoke plumes were also seen by CALIPSO over the UK on the morning of the 20th, Figure 5, with a layer measured over a vast area between the English South coast and Northern Scotland at between 10 km and 15 km in altitude. The origin of this layer from Canada was confirmed using the easily accessible, web-based, HYSPLIT trajectory model.
Trace gas concentrations have been significantly enhanced over Canada during this fire period. As expected levels of CO have been greatly enhanced over Northern Canada and into the Arctic region (Figure 6). The fires have been so intense that it has allowed other species to be detected which are usually in concentrations below the noise level of the IASI instrument. Two such example are C2H4 and HCOOH which are shown in elevated concentrations in figure 7.
By late August 2017 the fires were beginning to recede but the effects from the fires and atmospheric pollution is only starting to be understood. The challenges over the next few months which NCEO will be looking at is using the satellite measurements in conjunction with Earth system models. A particular focus is likely to be studying the effects of the injection of a number of pollutants directly into the stratosphere and on the radiative forcing of climate due to the increase in aerosols.