Climate strongly influences global wildfire activity, and recent wildfire surges may signal fire weather-induced pyrogeographic shifts. Here we use three daily global climate data sets and three fire danger indices to develop a simple annual metric of fire weather season length, and map spatio-temporal trends from 1979 to 2013. We show that fire weather seasons have lengthened across 29.6 million km 2 (25.3%) of the Earth's vegetated surface, resulting in an 18.7% increase in global mean fire weather season length. We also show a doubling (108.1% increase) of global burnable area affected by long fire weather seasons (41.0 s above the historical mean) and an increased global frequency of long fire weather seasons across 62.4 million km 2 (53.4%) during the second half of the study period. If these fire weather changes are coupled with ignition sources and available fuel, they could markedly impact global ecosystems, societies, economies and climate.

[1] Forty-four small-scale experimental fires were conducted in a combustion chamber to examine the relationship between biomass consumption, smoke production, convective energy release, and middle infrared (MIR) measurements of fire radiative energy (FRE). Fuel bed weights, trace gas and aerosol particle concentrations, stack flow rate and temperature, and concurrent thermal images were collected during laboratory-controlled burns of vegetative fuels. Using two different MIR thermal imaging systems, measurements of FRE taken at polar angles of ff48°and ff60°were found not to be significantly different from each other (p < 0.05), but were significantly different from those obtained at ff76°. A simple linear regression revealed that less than 12% of the variation in biomass consumption remained unexplained by the measured FRE regardless of MIR sensor characteristics, fuel type, or viewing angle. Measurements of FRE detected per unit of dry organic material consumed ranged from 1.29 to 4.18 MJ/kg, corresponding to an average of 12 ± 3% of the higher heating value of the biomass. Whole-fire emission factors agreed with previously reported values, and emission ratios relating total mass production to FRE were determined for CO 2 , CO, NO, NO 2 , and particulate matter less than 2.5 mm in aerodynamic diameter. A heat balance performed on the system showed that the release of convective energy could be predicted from a measurement of FRE (r 2 ! 0.84), and together these two modes of heat transfer accounted for 61 ± 13% of the total, potential heat of combustion available in the preburn solid fuel.Citation: Freeborn, P. H., M. J. Wooster, W. M. Hao, C. A. Ryan, B. L. Nordgren, S. P. Baker, and C. Ichoku (2008), Relationships between energy release, fuel mass loss, and trace gas and aerosol emissions during laboratory biomass fires,