環境：オーストラリアの山火事による二酸化炭素排出量の急増と藻類の大増殖

The 2019–2020 Australian wildfires released more than twice as much CO₂ as previously reported on the basis of different fire inventories, reports a Nature paper. An independent study also published in Nature suggests that aerosol emissions from these wildfires are likely to have fuelled vast plankton blooms thousands of kilometres away in the Southern Ocean. The findings highlight the complex links among wildfires, ecosystems and the climate.

Climate-change-driven droughts and warming play a role in increasing the frequency and intensity of wildfires, which release CO₂ into the atmosphere, potentially driving further climate change and increasing the risk of wildfires. In the summer season of 2019–2020, around 74,000 km2 — an area roughly equivalent to 2.5 times the area of Belgium — burned in the eucalyptus forests in the coastal regions of Victoria and New South Wales, Australia. These wildfires are known to have been extremely large in scale and intensity, and to have released large amounts of CO₂ into the atmosphere, but emission estimates remain uncertain.

To gain insights into the amount of CO₂ released by the 2019–2020 Australia wildfires, Ivar Van der Velde and colleagues analysed new high-resolution satellite measurements of carbon monoxide concentration in the atmosphere, from which they infer fire-induced carbon emmisions. They estimated that around 715 teragrams of CO₂ were emitted between November 2019 and January 2020 — more than twice the amount previously estimated from five different fire inventories and comparable to a bottom-up bootstrap analysis of this fire episode, surpassing Australia’s normal annual fire and fossil fuel emissions by 80%. Gaining a stronger understanding of the atmospheric burden of CO₂ caused by these fires, and what they will cause in the future, is critical for constructing future scenarios of the global carbon balance, the authors note.

In addition to carbon emissions, wildfires release aerosols that affect ecosystems; for example, transportation of nutrients such as nitrogen and iron can enhance plankton growth. Nicolas Cassar, Weiyi Tang and colleagues report the presence of extensive phytoplankton blooms from December 2019 to March 2020 in the Southern Ocean, downwind of the fires. Aerosol samples originating from the fires contained high levels of iron, which the authors suggest were transported vast distances and fertilized the blooms.

Together, these studies demonstrate that wildfires can have an important influence on atmospheric CO₂ levels and ocean ecosystems.