By Beatrice Omotosho

Volcanic eruptions

Volcanic eruptions release small fine particles of SO₂ into the atmosphere. Large, explosive eruptions in particular such as the 1991 Pinatubo eruption, cause these particles to reach the stratosphere. These particles often linger for about 1-5 years and move around the globe as it reacts with the atmosphere forming sulphuric acid, which causes increased reflection of solar radiation and therefore cooler earth temperatures, resulting in a small average decrease in global mean temperatures; as if it wasn’t cold enough over here. The 1991 eruption, caused a 2 per cent reduction in the solar radiation that reached the lower atmosphere, reporting of spectacular sunsets globally for months due to the dust, and lower average global temperatures by a quarter of a degree Celsius persisting for two years.

Volcanic aerosols can also facilitate the destruction of stratospheric ozone due to heterogeneous chemical reactions which reduces the amount of UV absorption and in turn reduces lower stratosphere radiative heating, (For a visual representation, see image 1). These eruptions can therefore be said to be primary drivers of natural variability in climate due to their short term regional-global scale cooling effects. We can see in figure 1 the effects of 3 major eruptions on global mean temperatures. At each point following an eruption, there was a reduction in average temperature, then the trend resumes, hence, eruptions can account for the oscillations in the climate trend, but not the trend itself, they only affect the overall climate a little and for a short time. Boom, there goes another of your sceptic’s arguments. At this point, I imagine they are sinking into their chair in front of you.

Southern oscillations; El Niño and La Niña:

Winds most often drag the surface of the sea long with them, just like bad friends will drag you along with them. Trade winds in the tropical pacific moving from east to west in the same direction as ocean currents, deposit warm piles of water west of the pacific and cooler water in the east. A weakening of trade winds causes a reverse in the current. El Niño is therefore the reverse in these currents due to weak trade winds which causes a persistence of a large body of warm water in the east, resulting in El Niño events which tend to occur around Christmas, (hence its name), with a frequency of around every 3-5 years. During El Niño years, huge amounts of energy is released by the oceans therefore increasing global air temperatures, and affecting global weather, bringing about heavy rains, floods, declines in the fishing industries, droughts and in turn, forest fires, and just about every other bad thing you really need something you can blame for e.g my grades.

There are many non-linear characteristics in earth systems, therefore we at times see different feedback effects. El Niño years results in higher temperatures and higher temperatures/climates increase the frequency of these same events therefore it is possible to see some sort of positive dynamical feedback (PDF) effect which in turn could cause an even greater effect on global climate. Arguably, the growth rate of El Niño vs La Niña is due to “enhanced sensitivity of the ocean’s dynamic response to wind stress”, which is key to illustrate the increase in PDF that comes from the nonlinearity of response between atmospheric pattern and “sea surface temperature (SST) anomalies”.

La Niña is essentially the opposite of El Niño, where we see an intensification of the natural/normal pattern of warm waters in the west and cool in the east. Very strong trade winds cause a deeper layer of warm water in the western pacific and shallower cooler water in the eastern side (Philander, 1985). This causes lower sea surface temperatures and less energy being released by the oceans and therefore cooler than average temperatures during these years. The period between both of these events are highly irregular, although arguably more El Niño events are likely to be seen due to the warming climate.

Naturally occurring GHG

Natural variations in levels of CO2 have been linked to climate change for a long time and this is accounted for by the evidence collected from Antarctic ice cores; honestly, this must be like me trying to fit into size 8 jeans, almost impossible. Figure 2 shows the close positive relationship between CO2 concentrations, (and some other gases) and Antarctic temperatures (with variations in deuterium as a proxy) over the last 600,000 years. We see oscillations in concentration levels largely due to the pronounced natural seasonal cycle of northern hemisphere plant productivity, vegetation and crop cycles, growth vs dieback etc.

Recently, the trend has taken a very sharp and steep turn, with the increase in CO2 levels and temperatures too high to be attributed to natural oscillations of atmospheric CO2 but only to dramatic increase in anthropogenic GHG. Of the points mentioned so far, including in part 1 and 2 of this series, volcanic eruptions and El Niño events are the two largest natural contributors to fluctuations in recent climate records, Figure 3, illustrates this clearly with the effects of volcanic and El Niño years evident on the graph. I’m sure by now, you could do a mic drop and victoriously walk away from the sceptics, but just for those stubborn ones, in the final part of this series, we are going to take things just a little further.