Climate change and boreal forest fires. Climate change is an accepted reality, and observed and forecast impacts are greatest at northern latitudes and over land, particularly over the more continental regions of Canada , Russia and Alaska . These are areas where large fires have been common since the last Ice Age, and recent research (e.g. Stocks et al. 1998; Flannigan et al. 2003) indicates that more frequent and severe fires are expected as the climate changes. This will have a significant impact on the age class structure and carbon budget of the boreal/Arctic zone in particular and the globe in general. Boreal fires consume large quantities of fuel and spread quickly, creating high energy release rates that are often sustained for long burning periods. This frequently results in convection columns with strong vertical development that reach beyond the tropopause. Long-range smoke transport from large boreal fires is already common, with smoke loads from Siberian fires often reinforcing smoke from North American fires. This phenomenon is expected to become even more common with more frequent and severe fires in the future, increasing the likelihood that smoke from boreal fires will provide a positive feedback to climate change (Kurz et al. 1994).

Long-range transport of boreal forest fire emissions. Large amounts of smoke and trace gases emitted by boreal forest fires can be subject to considerable vertical and horizontal transport (e.g. Stocks and Flannigan, 1987; Siebert et al., 2000; Fromm et al., 2005). Fire emissions can travel over continental (Wotawa and Trainer, 2000), intercontinental (Forster et al., 2001; Honrath et al., 2004), and even hemispheric (Damoah et al., 2004) distances. Recent satellite observations and lidar measurements observed substantial amounts of forest fire smoke in the tropopause region and lower stratosphere at high latitudes and in the Arctic region (e.g. Waibel et al., 1999; Fromm et al., 2000; Damoah et al, 2004). Especially over snow/ice surfaces the short-wave reflectivity to space can be considerably reduced by forest fire smoke, which may have important implications for the radiative energy budget in the polar region (Hsu et al., 1999). Episodically, the fires also pollute large regions in the lower troposphere at high latitudes (Forster et al., 2001), but unfortunately few measurements in the Arctic free troposphere (e.g., during the ABLE 3A and 3B campaigns; Harriss et al., 1994, Shipham et al., 1992) and at the surface (e.g., Ily-Tuomi et al., 2003) exist. There is clear evidence for deposition of ammonia from biomass burning sources in Arctic ice core measurements (Whitlow et al., 1994). Presumably, the deposition of substances like soot from such fires may decrease the albedo of ice and snow and lead to enhanced melting of Arctic glaciers and sea ice (Kim et al., 2005). However, to date no data exist to reliably establish such a connection.

Pyro-convection. An exciting new development with respect to climate change is the recent discovery of transport of biomass burning emissions into the lower stratosphere through an explosive combination of intense forest fires and extreme convection (see Figure 4). This “pyroCb” source of stratospheric injection has been observed remotely (lidar, balloon sounding, and satellite solar occultation) (e.g. Fromm et al., 2000; Fromm and Servranckx, 2003) and in-situ (e.g. Jost et al., 2004). Although boreal fire research scientists have reported forest fire convection columns above 13 kilometres in height (e.g. Stocks and Flannigan 1987), recent publications and unpublished data paint an emerging picture of the pyroCb phenomenon as a recurring one, with hemispherical impact (e.g. Fromm et al., 2005). UTLS enhancements, attributable to pyroCb, of aerosols, carbon monoxide, ozone, and acetonitrile have all been observed. While observations clearly show that deep upward transport of biomass burning emissions into the upper troposphere and lower stratosphere is frequent (e.g., Nedelec et al., 2005), the mechanisms are poorly understood. Factors that could enhance convective uplift over the fires are the heat and water vapour released by the fire, microphysical cloud processes (Andreae et al., 2004), and probably radiation absorption by soot particles above the cloud tops and in the stratosphere.

The highest altitude where forest fire smoke was observed in situ was 17 km (remote sensing observations exist even at higher altitudes), several kilometers above the tropopause and at potential temperatures greater than 380 K (Jost et al., 2004), thus in a region that is commonly referred to as the stratospheric overworld. The chemical impact of the forest fire emissions at such high altitudes is unknown. Both efficient ozone formation as well as severe ozone destruction are possible scenarios. Furthermore, the stratospheric residence time of aerosols may in fact be long enough to affect stratospheric ozone depletion during the following winter/spring.

Due to their proximity to the Arctic , their large source strength, and the special processes accompanying them as described above, boreal forest fires need special attention during POLARCAT. For this, an integrated study using low- as well as high-flying aircraft, satellite measurements, and models is needed. The launches of AURA, CALIPSO and METOP come at an opportune point to observe these processes and also provide critical data needed for the flight planning. In addition, satellite data like TOMS aerosol index, solar/lunar occultation profiles (e.g. POAM III, SAGE II and III, and GOMOS), MODIS imagery, MOPITT, IASI, ACE, NOAA POES and GOES imagers will be used to provide forecast guidance for potential fire blowup conditions worldwide, enabling also targeted operating modes for certain satellite instruments.

POLARCAT objectives related to boreal forest fires.