1. Introduction
Forests play an important role in the global ecological, environmental and recreational functions [
1,
2]. It is well recognized that the forests can absorb atmospheric carbon, regulate rainfall, moderate temperature, and restrain soil erosion [
3,
4]. To a large extent, the forest fire is a dominant disturbance factor in almost all the forest vegetation zones throughout the World, and it is considered to be a potential hazard causing physical, biological, and environmental consequences [
5–
8]. Furthermore, forest fires can have adverse societal impacts regardless of whether they are caused by natural forces or human activities.
The forest resources are very limited in China, and only account for 20.36% of the whole national’s territory [
9]. According to the Seventh National Forest Resources Inventory during 2004 to 2008, the forest area was estimated at 195 million ha and its stock volume was 13.72 billion cubic meters, ranking fifth in the world after Russia, Brazil, Canada, and the U.S.A [
9,
10]. It was reported that the forest fires burn an average of 0.86% of the total forest area every year in China [
11]. Temporally, over the past 40 years, the number of forest fires has increased significantly [
12]. During the period from 1950 to 1997, about 676,000 forest fires occurred and the average burned area reached about 822,000 ha each year [
13]. One of the largest forest fires occurred in the Daxing’anling Forest of the Northeast China during the period from 6 May to 2 June, 1987. The burned area was in excess of 1.3 million ha of the prime forest, resulting in the loss of over 200 lives and over 50,000 homes [
14]. As a result, the frequent occurrence of forest fires has become an important issue in view of the forest degradation in China [
11,
15–
17].
Early detection and real-time monitoring of fire activities are essential for fire management and planning. Given the high cost and potential hazards associated with the ground-based and air-borne fire monitoring, space-borne satellite sensors have been widely used to detect and monitor forest fires in recent years [
18–
21]. Earth observations provide useful data for developing consistent method for detecting area and severity of active burning at national, continental or global scales. Sensors onboard many satellites with multi-spatial and multi-spectral resolutions, such as NOAA/AVHRR (the National Oceanic and Atmospheric Administration/the Advanced Very High Resolution Radiometer) [
19,
20], EOS-MODIS (Earth Observing System-Moderate Resolution Imaging Spectroradiometer) [
21–
24], Landsat ETM (Enhanced Thematic Mapper) [
25–
28] were designed to measure reflected and emitted radiation in the visible (VIS, 0.4–0.7 μm), the near infrared (NIR, 0.7–1.3 μm), the short-wave infrared (SWIR, 1.3–8 μm), the middle infrared (MIR, 3–8 μm), and the thermal infrared (TIR, 8–13 μm).
The satellite sensors such as NOAA/AVHRR, Chinese FY (FengYun)-series, MODIS, CBERS (China-Brazil Earth Resources Satellite), and ESA (European Space Agency) ENVISAT (Environmental Satellite) have been widely applied to detect forest fire hot spots and burned areas in China [
29]. The relatively high temporal and spatial resolution data acquired by the polar orbiting satellites, such as NASA EOS Terra/Aqua, and Chinese FY polar orbiting series, increase the likelihood that cloud-free data can be obtained for mapping the distribution of the large-scale burned areas. In contrast, the Geostationary platforms, such as NOAA GOES (Geostationary Operational Environmental Satellite), MTSAT (Meteorological Satellite), and Chinese FY geostationary orbiting series satellites can provide continuous observations over large geographic regions, making it possible to detect the short duration fires and also to resolve the diurnal behavior of large fires [
29].
Biomass burning is a major contributor of particulate matter (PM) and trace gases to the global troposphere [
30,
31]. Recent studies indicated that the global interannual variability in terrestrial ecosystem fluxes and in the atmospheric CO
2, are strongly influenced by forest fire emissions [
32,
33]. It is noted that China’s forests have greatly contributed to the carbon sink in the global carbon budgets during the last five decades [
34]. However, recent increases in forest fires in China have already caused an increase in CO
2 emissions [
35,
36].
The forest fire risk prediction is very important for fire management and fire protection strategies planning. The occurrence and spread of forest fire are basically determined by the fuel property, weather conditions, and terrain features. A prerequisite for the strategy planning is the ability to identify forest fire risk zones across both broad areas and local sites. Forest fire risk zones are locations where a fire is likely to start, and from where it can easily spread to other areas [
37].
In this paper, we primarily present an overview on the status of detecting forest fire hot spots and fire areas by using satellite sensors in China over the past three decades, and analyze a few developed algorithms used for detecting fires with greater accuracies. We illustrate a forest fire emission model used to estimate gas and aerosol emissions in China. We also evaluate forest fire risk potential models that have been used for predicting the forest fires occurrence in China.
4. Forest Fire Emissions Estimation in China
Biomass burning emissions including greenhouse gases such as CO
2, CH
4 and aerosol particles have significant influences on air quality, atmospheric chemical composition, the earth’s radiation budget, and global climate change [
75–
79]. In recent years, the forest fires have significantly aggravated and affected national forest ecosystem due to human activities and some extreme climate events. The forest fires are believed to have generated much carbon emissions in China [
35,
36]. An approach for addressing biomass burning emissions is to accurately quantify the amount of emissions spatially and temporally based on observations of active fires.
Figure 6 presents a flowchart of the Chinese Forest Fire Emission Estimation System (named CFFEES).
Figure 6 shows the forest fire emissions estimation system in China. In the CFFEES, the forest fire burned area (BA) and fuel loading (FL) detection by using multi-sensors including NOAA/AVHRR, FY-series, EOS-MODIS, Landsat-TM, ENVISAT
etc.; and which are integrated with forest type (or land use), forest inventory data, the fraction of fuel consumed (FF) in live vegetation data, and the associated emission factor (EF). The CFFEES may calculate the forest fire emissions of various gases (
i.e., CO
2, CO, CH
4) or PM by using the emission model showed in
Figure 6. All the source data are spatially and temporally distributed and daily assimilated according to the forest burning hot spot defined by using the satellite observations. The entire estimated emissions of forest fire using the CFFEES system can be calibrated by using the flux tower observation data.
4.1. Forest Biomass Simulation Based on BEPS Model and Satellite data
In the CFFEES model, it is important to have an accurate estimation of forest biomass (or fuel loading). Brown (1984) started to estimate biomass of a tropical forests based on the forest volumes [
80]. Recently, many bio-ecological models using remote sensing data as input have been used to estimate the net primary productivity (NPP) and biomass [
81–
85]. Liu
et al. developed a boreal ecosystems productivity simulator (BEPS) model based on the Forest BioGeochemical Cycles (FOREST-BGC) model for quantifying the biophysical processes governing ecosystems productivity [
81]. The BEPS model has been used to accurately estimate forest NPP in two forest sites of Northeast China and one forest site in Western China when using the remotely sensed data of high spatial resolution as input. The results show that the observed NPP agrees well with the modeled NPP from BEPS [
86,
87]. Others examples in China also support that the estimated forest NPP are highly related to field data in the study area by using BEPS model [
88–
91].
4.2. Quantifying Emission of Forest Fire in China using Satellite Data and Emission Model
Forest fire is a major process and contributes to an increasing concentration of pollutants in the atmosphere, such as CO
2 CO, CH
4, and aerosols in China [
35,
36]. Tian
et al. estimated the amount of carbon emissions directly from forest fires in China during 1991 to 2000 using satellite data and an emission model [
35]. It was concluded that during this period China’s forest fires emitted CO
2 74.2–104.7 Tg, CH
4 1.797–2.536 Tg, and smoke aerosols 0.999–1.410 Tg. During 1991–2000, the average CO
2 and CH
4 emissions of forest fire accounted for 2.7%–3.9% and 3.3%–4.7% of the total CO
2 and CH
4 emissions, respectively.
For examining the spatial and temporal patterns of the fire-induced carbon emissions, Lü
et al. [
36] estimated the emission of carbon (C) and carbon-containing trace gases including CO
2, CO, CH
4, and nonmethane hydrocarbons (NMHC) from forest fires in China from 1950 to 2000 by using a combination of remote Sensing (Landsat TM/ETM images) [
92], the forest fire inventory, and a terrestrial ecosystem model. The results showed that mean annual carbon emission was about 11.31 Tg from forest fires in China, ranging from a minimum of 8.55 Tg to a maximum of 13.9 Tg. This amount of carbon emission resulted from the atmospheric emissions of four trace gases,
i.e., 40.66 Tg CO
2 with a range from 29.21 to 47.53 Tg; 2.71 Tg CO with a range from 1.48 to 4.30 Tg; 0.112 Tg CH
4 with a range from 0.06 to 0.2 Tg, and 0.113 Tg NMHC with a range from 0.05 to 0.19 Tg [
36]. Wang
et al. [
93] studied the influence of fire on carbon distribution and net primary production of boreal forests in north-eastern China, the result showed that the 1987 conflagration in north-east China released 25–49 Tg C to the atmosphere. Information on the amount of emission by forest fires should help the investigation on the transportation and dispersion studies of the plumes, and also provide important input data to the climate models.
6. Conclusions and Remarks
This paper has provided an overview of the fire detection, emission estimation and risk prediction via operational satellite sensors, simulated models and climate information in China in the past three decades. Overall, in the last 30 years, China has developed its meteorological and environmental satellite sensors to detect forest fires across the nation and the surrounding areas. The second generation of China’s polar orbiting meteorological satellite (Fengyun-3) with substantially advanced multi-spectral imaging systems was launched on 28 May, 2008. The disaster and environment monitoring and forecast small satellite constellation was launched on 6 September, 2008, and comprised two satellites (HJ-1A and HJ-1B). There are three kinds of optical remote-sensing sensors on board, multi-spectral imager (CCD) with 360 km swath and 30 m spatial resolution, infrared imager (IRS) with 720 km swath and 150 m spatial resolution, and hyper-spectral imager (HIS) with 50 km swath and 100 m spatial resolution, and 110 channels range 0.45–0.90 μm [
116]. The CCD (0.43–0.9 μm) and IRS data (3.5–3.9 μm) work well for the hot-spot detection and forest fires assessment [
116–
118]. Along with other currently operational satellites such as NOAA/AVHRR, FY-series, EOS/MODIS, ENVISAT and CBERS, a more accurate and efficient operational system in fire detection, emission estimation and risk prediction can be established and operated before, during and after a fire season. However, for the fire risk management one needs to take into account and assess the fire risk potentials.
A forest fire can be a real ecological disaster regardless of whether it is caused by natural forces or human activities. It is possible to map forest fire risk zones to minimize the frequency of fires and avert damage. As climate changes and extreme climatic events become more prevalent, the frequency and intensity of forest fire occurrence will likely increase in China and the surrounding areas. It is important to develop a new fire danger and risk model to forecast the forest fire potential. Accurate forecasting of the El Niño event occurrence is critical.
Forest fires have significant impact on vegetation dynamics and deplete timber resources, so that forest fires monitoring is a critical aspect of sustainable forest management. Forest fires have direct and important effects on the global carbon cycle, atmospheric chemistry, regional radiation and climate and in regulating terrestrial ecosystems and biodiversity [
119]. Uncertainty over the effects of future climate change upon fire regimes, and the importance of vegetation-atmosphere feedbacks has fostered increased effort to develop coupled models of vegetation and fire to understand these future changes. To reach a closure in carbon balance including some greenhouse gases as CO
2 and CH
4,
etc., and to understand smoke aerosols produced from burning and which influence global climate and atmospheric chemistry, the forest fire emissions estimates based on satellite sensors is very important and complex. Additional studies should be carried out to accurately detect forest burned areas, fragmentation of burned scars, fuel loading and biomass based on the multi-sensors. Further investigation, analysis, and computations of the fraction of fuel consumed by forest fire, and emission factors for the different vegetation types are necessary to quantitatively evaluate and verify Chinese forest fires emissions in the future.