How Volcanoes affect the Climate
Volcanoes have a significant and complex affect on the climate across the world.
A single volcanic eruption actually creates a cooling effect that can last for several years. This is due to the sulphur and ash emitted during an eruption: the ash reaches the upper atmosphere and partially blocks out solar radiation to the Earth's surface for a between 6-12 months before falling back to earth, whilst the sulphur reacts with atmospheric water and oxygen, creating a short term cooling effect for a few years, producing sulphuric acid which then falls as acid rain.
However, whilst sulphur remains in the atmosphere for a few years, carbon dioxide remains in the atmosphere for hundreds of years. So in an era when thousands of volcanoes were erupting, the high amount of carbon dioxide being produced and remaining in the atmosphere from multiple volcanoes countered the cooling effects of the sulphur from each of the volcanoes.
Volcanoes - Mount Pinatubo
In 1991, the eruption of Mount Pinatubo in the Philippines affected the worldwide climate substantially. A global layer of sulphuric acid haze formed, reducing the normal amount of sunlight reaching the earth's surface by approximately 10%. At ground level, global temperatures dropped by 0.4°c, whilst global stratospheric temperatures rose by several degrees. Ozone in the stratosphere was significantly depleted.
The sulphuric acid haze lasted three years, suppressing ground level global temperatures and increasing stratospheric temperatures for the whole of this time.
Today, eruptions the size of Mount Pinatubo occur a few times every century. But by Cretaceous standards, the Mount Pinatubo eruption was comparatively small and insignificant.
Huge volcanoes eruptions occur only a few times every hundred million years but can reshape the climate for millions of years and cause mass extinctions. Significant volcanic activity is one of the two theories as to why the dinosaurs became extinct.