Talk:Nuclear winter

Eruption of Mt. Pinatubo and agriculture

The eruption of the Philippines volcano - Mount Pinatubo in June 1991 ejected roughly 10 km³ (2.4 cu mi) of magma and "17,000,000 metric tons"(17 teragrams) of sulfur dioxide SO₂ into the air, introducing ten times as much total SO₂ as the 1991 Kuwaiti fires,^[1] mostly during the explosive Plinian/Ultra-Plinian event of June 15, 1991, creating a global stratospheric SO₂ haze layer which persisted for years. This resulted in the global average temperature dropping by about 0.5 °C (0.9 °F).^[2] As volcanic ash falls out of the atmosphere rapidly,^[3] the negative agricultural effects of the eruption were largely immediate and localized to a relatively small area in close proximity to the eruption, as they were caused by the resulting thick ash cover that resulted.^[4][5] Globally however, despite a several-month 5% drop in overall solar irradiation, and a reduction in direct sunlight by 30%,^[6] there was no negative impact to global agriculture.^[7][8] Surprisingly, a 3-4 year^[9] increase in global Agricultural productivity and forestry growth was observed, excepting boreal forest regions.^[10]

The means by which this was discovered, is that initially at the time, a mysterious drop in the rate at which carbon dioxide (CO₂) was filling the atmosphere was observed, which is charted in what is known as the "Keeling Curve".^[11] This led numerous scientists to assume that this reduction was due to the lowering of the Earth's temperature, and with that, a slow down in plant and soil respiration, indicating a deleterious impact to global agriculture from the volcanic haze layer.^[7][12] However upon actual investigation, the reduction in the rate at which carbon dioxide filled the atmosphere did not match up with the hypothesis that plant respiration rates had declined.^[13][14] Instead the advantageous anomaly was relatively firmly^[15] linked to an unprecedented increase in the growth/net primary production,^[16] of global plant life, resulting in the increase of the carbon sink effect of global photosynthesis.^[7][17] The mechanism by which the increase in plant growth was possible, was that the 30% reduction of direct sunlight can also be expressed as an increase or "enhancement" in the amount of diffuse sunlight.^{[7][13][18][19]}

With, owing to its intrinsic nature, can illuminate under-canopy leaves permitting more efficient total whole-plant photosynthesis than would otherwise be the case.^[7][20] In stark contrast to the effect of totally clear skies and the direct sunlight that results from it, which casts shadows onto understorey leaves, strickly limiting plant photosynthesis to the top canopy layer.^[7][21] This increase in global agriculture from the volcanic haze layer also naturally results as a product of other aerosols that are not emitted by volcanoes, such as man-made "moderately thick smoke loading" pollution, as the same mechanism, the "aerosol direct radiative effect" is behind both.^[10][22][23]