According to a new study from researchers at the Lawrence Livermore National Laboratory (LLNL) volcanic activity may be responsible for the so-called global warming ‘hiatus’. That may not seem logical at first glance. After all, according to recent studies volcanic activity may be what warmed Mars enough to make water flow on the surface. Is it possible for the same phenomenon to be responsible for global cooling and global warming? The answer, like most things related to climate is ‘it’s complicated’.

First it is important to understand what is meant by a global warming ‘hiatus’. For many years, prior to 1998, global temperatures were increasing at roughly the same rate as greenhouse gasses in the atmosphere. After 1998 the ‘hiatus’ started but in this case hiatus does not mean that temperatures stopped going up. Temperatures simply stopped going up as fast as they had been.

“This so-called warming hiatus began around 1998, following a particularly strong El Niño that year. Between 1951 and 2012, the average global temperature rose by 0.12°C per decade, compared with just 0.05 °C for the period from 1998 to 2012.” according to Nature.

So, despite the hiatus, 2014 was still the hottest year on record.

Next it is important to understand that when a volcano erupts a number of gasses, including carbon dioxide, sulfur dioxide and hydrogen sulfide are released and these gasses have different short and long term impacts on climate.
Carbon dioxide (Co2) is a greenhouse gas that contributes to climate change.

Sulfur dioxide (So2) interacts with oxygen in the atmosphere, forming droplets which reflect sunlight away from the earth and causing a temporary cooling effect. However So2 also contributes to acid rain, ozone depletion and acid rain.

Hydrogen sulfide (H2S) is another toxic pollutant that can cause a number of health impacts in humans and animals.

So, while volcanic activity can cause a short term cooling effect on the environment, the overall impact is not necessarily good for the environment.

It is also worth noting that there is evidence that volcanic activity increases as global temperatures rise.

In a paper published in Geophysical Research Letters, research from LLNL suggest that a series of small volcanic eruptions over the last 15 years may account for the hiatus. The study relied on a combination of air, ground and satellite measurements and demonstrated that the amount of reflected sunlight was considerably higher than previously believed.

“This new work shows that the climate signals of late 20th- and early 21st-century volcanic activity can be detected in a variety of different observational data sets,” said Benjamin Santer, a Lawrence Livermore National Laboratory scientist and lead author of the study in a statement.

Previous hypothesis indicated that weak solar activity and increased heat absorption by the oceans could account for the slow down in global warming. The link between warming and volcanic activity was originally suggested in a study by Susan Solomon of the Massachusetts Institute of Technology in a paper published in the journal Science in 2011.

Prior to 2011 it was believed that only very large events, such as the 1991 Mount Pinatubo eruption which ejected 20 million metric tons of sulfur into the atmosphere, could impact global climate.

The authors of the most recent paper however found the evidence they needed between atmospheric layers. Satellite measurements of the sulfuric acid droplets between the stratosphere and the troposphere, where weather takes place, provided evidence of volcanic cooling. The researchers believe that sulfur could be responsible for cooling of 0.05 degrees to 0.12 degrees Celsius since 2000.

The researchers were able to identify volcanic signatures from late 20th and early 21st eruptions in atmospheric temperature, moisture and the reflected solar radiation at the top of the atmosphere.

“The fact that these volcanic signatures are apparent in multiple independently measured climate variables really supports the idea that they are influencing climate in spite of their moderate size. If we wish to accurately simulate recent climate change in models, we cannot neglect the ability of these smaller eruptions to reflect sunlight away from Earth,” said Mark Zelinka, another LLNL author.

While these eruptions may cause a short term cooling effect, leading to the global warming hiatus the long term impact of volcanic activity is more Co2 in the atmosphere along with other negative environmental effects.