If Mt Agung erupts

Big volcanic eruptions temporarily reduce global temperatures because they release sulphur high into the atmosphere, which combines with water to produce sulphuric acid aerosols which reflect incoming sunlight, thus cooling the earth.  If the eruption is large enough, the cooling effect can last a couple of years. There's an excellent article about it on Carbon Brief.

The last time Mt Agung erupted, in 1963, it reduced global temperatures by 0.2 C, as you can see in the chart below.   The blue line in the chart suggests the possible impact on the underlying upward trend if (when) Mt Agung erupts.  No doubt when this happens, denialists will argue that global temperatures will have stopped rising and we can stop worrying about carbon emissions.  Which will be nonsense.

Source: Zeke Hausfather at Carbon Brief

But it does raise the question:  could we simulate volcanic eruptions by artificially releasing sulphur dioxide at very high altitudes (18 kms plus) which will help offset the impact of global warming? This is called geo-engineering. 

There are questions about geo-engineering: cost (who will pay? will it be cheaper than just switching to a green economy? At what scale would we need to do it? ); environmental impact (remember acid rain? is it safe to have a continuous layer of sulphuric acid in the atmosphere?); and politics (would it stop attempts to reduce carbon emissions?)

The Economist has done a short video on it, which you might find interesting.