On June 15, 1991 Mt. Pinatubo in the Philippines erupted over the course of 9 hours. That single eruption had a measurable impact on the Earth’s temperature for three years after the eruption. This is a very good case for studying how quickly the Earth’s temperature responds to changes in the climate system. The time that it takes for the Earth to respond matters a great deal in the climate debate. A quick response would indicate that even if CO2 does cause warming of the Earth’s climate, the effects are felt quickly and and not delayed far into the future. This would indicate that the full effects of 390 ppm of CO2 are in full effect and that the warming is already fully realized for the current emissions of CO2.
The eruption is useful for measuring the time lag because it is an event that happened at a single instance and then stopped. In engineering terms there was a step (or a single pulse) function change to the Earth’s atmospheric system on that date in June. The peak altitude of the eruption was 34 km (21 miles). The height of the eruption is what allowed the SO2 to reach the stratosphere. Once there it was able to slowly spread out and cover the entire Earth at the stratospheric level. This is why the eruption of Mt. Pinatubo had such an impact on the Earth’s climate. If the gas stayed in the much more humid troposphere it would have mixed with water and dissipated from the atmosphere much more quickly. Since it was in the dry stratosphere it stayed there for much longer.
This is evident in that the eruption only released 17 million tons of SO2. While that seems significant, it is comparable to the amount of SO2 released annually by the United States in 1991. The only difference is altitude. Much of the gas from the eruption reached the upper atmosphere and so it was able to spread globally there. That is why injecting SO2 into the stratosphere is one of the ideas floated by warmists to combat global warming.
Once in the stratosphere the SO2 did slowly mix with water vapor to form H2SO4 (sulfuric acid), but in the stratosphere there is no precipitation to allow for it to be quickly removed. So it stayed there in a mix of SO2 and H2SO4. Satellite studies show that until the end of July 1991 the effects stayed mostly in the equatorial region of the world, but after that the cloud spread further and further around the world. By early 1992 the concentration of SO2 and H2SO4 had increased around the world in the stratosphere to a small degree. By June of 1992 after a full year the cloud of sulfur gases had completely blanketed the Earth. This is also evident as the winter of 1992 saw the largest ozone hole ever recorded in Antarctica.
As the tropics had the gas cloud first it was the first to show a significant temperature effect. From June to Oct of 1991 the temperature anomaly of the tropics decreased by 0.7 °C. The concentration in the tropics decreased as the gas spread to other portions of the atmosphere. This helped the temperature in the tropics recover to normal anomalies. The rest of the world experienced the main cooling as the concentration of the gases helped reduce the amount of energy from the sun that reached the ground.
As the gas cloud reached its maximum size and concentration a year after the eruption, the strongest effect of the cooling was felt. July-September of 1992 showed a world that was about 0.5 °C cooler than the year before. The rate of temperature change for the 13 month period from the eruption was -0.03 °C/month. A rate of cooling that strong over a 5 year period would drop the Earth’s temperature by almost 2 °C. That effect was felt as quickly as the gas cloud spread from the point of the eruption. The time lag was negligible for this event.
The Earth also warmed up at the same rate that the gas cloud dissipated. In late 1993 the concentration of the cloud in the atmosphere was greatly weakening and that also shows in the global temperatures.
This is a great case study of how quickly the Earth responds to a change in the climate. Radiative Heat Transfer is a fast response mechanism and there is no basis for the idea that it would take decades for warming to take place for any given change to the greenhouse gases in the atmosphere. It did not take a decade for the Earth to cool from this eruption. It took 4 months for the cloud to spread around the tropics and drop the average temperature anomaly by 0.7 °C.
There is no evidence for the idea that it takes the Earth a long time to respond to changes in radiative energy. There is overwhelming evidence that the Earth changes within a matter of months to those changes. While this case does show that the Earth’s climate can be disrupted, it also shows that the response is almost instantaneous. The theory that it takes longer than a few months for warming to take place as a result of a change in the Earth’s atmosphere has no basis.