On the rebound - bouncing ever upwards

There's a recent article on WUWT by Paul Homewood.  He's complaining about the hottest decade on record, and writes this:

Of course, they might have mentioned that the eruption of Pinatubo in 1991 meant that temperatures in that decade were depressed.

This is what happened over the last few years:

Data Source: NASA GISS

Yes, you can see a dip in the surface temperature in 1992 and 1993, but consider what happened in 1998.  Overall you'd be hard-pressed to say that temperatures in the 1990s were depressed.

What happens with a volcano is that aerosols reflect sunlight.  After the aerosols have disappeared then things get back to "normal" as more sunlight once again penetrates the atmosphere.  Except that normal these days has increased greenhouse gases.

In times past, the volcano would have cooled things down for a few months and then earth would have warmed up again.  But it wouldn't have kept getting warmer.  It would merely have eventually returned to its pre-volcanic state.  Here is a NASA image illustrating what happens with a volcano:

Source: NASA

The NASA fact sheet describes it as follows:

Volcanic Aerosol
Three types of aerosols significantly affect the Earth's climate. The first is the volcanic aerosol layer which forms in the stratosphere after major volcanic eruptions like Mt. Pinatubo. The dominant aerosol layer is actually formed by sulfur dioxide gas which is converted to droplets of sulfuric acid in the stratosphere over the course of a week to several months after the eruption (Fig. 1). Winds in the stratosphere spread the aerosols until they practically cover the globe. Once formed, these aerosols stay in the stratosphere for about two years. They reflect sunlight, reducing the amount of energy reaching the lower atmosphere and the Earth's surface, cooling them. The relative coolness of 1993 is thought to have been a response to the stratospheric aerosol layer that was produced by the Mt. Pinatubo eruption. In 1995, though several years had passed since the Mt. Pinatubo eruption, remnants of the layer remained in the atmosphere. Data from satellites such as the NASA Langley Stratospheric Aerosol and Gas Experiment II (SAGE II) have enabled scientists to better understand the effects of volcanic aerosols on our atmosphere.

Paul Homewood writes:

This means that about half of the 0.21C warming was simply the rebound from Pinatubo, and nothing to do with greenhouse gases.

I can't see that being correct.  In 1998, temperatures rose way high because of the strong El Niño   So that more than made up for the low years earlier in the decade.

Here are the average for each decade from the 1960s onwards.

Data Source: NASA GISS

In his WUWT article, Paul points to his home page which in turn goes to this article by Dr Hansen.  Here is the figure from the Hansen article to which Paul ends up referring:

You can see that the temperatures dropped anyway in 1991 and stayed lower in 1992 moving back up in 1993.

What Paul is doing wrong is not recognising that the temperatures kept on going up after 1993 and 1994.  So no, it's not "half" of that warming that was a "rebound from Pinatubo".  What lifted the average of the 1990s was the very high temperature in 1998 from the El Niño .  What if Pinatubo hadn't erupted.  Well, I'd say it would have got hotter sooner.  The chart would have shifted a bit to the left.

There was no particularly low or particularly high temperature in the 2000s relative to any other year that decade.  2004 and 2008 registered the lowest annual average, but really it was hot for the entire decade.

Overall, the content and general tone of Paul Homewood's article came across as a typical denialist article trying to downplay global warming.



Here are some choice comments from WUWT:

Lew Skannen says:

July 6, 2013 at 1:56 am “Almost 94 per cent of countries logged their warmest 10 years on record, it said.”
I have no idea what the actual stats are but I can imagine a situation where this claim is made because Nauru, Lichtenstein, Andorra, Vatican, Mauritius, Luxembourg and half a dozen Pacific islands all had their warmest decade …. and Russia didn’t.

I don't know if Russia had the hottest decade on record, but I do know it had the worst heat wave on record in 2010.

Clive Bond of Brisbane says:

July 5, 2013 at 11:10 pm  The planet has been warming for 300 years since the Little Ice Age. When would the warmest decade be?

That's the point, Clive Bond.  It's called global warming and it's caused by human activity.  And it looks as if the deniers are starting to shift the goal posts again with this comment from AlexS who says:

July 5, 2013 at 3:40 pm  It is telling they don’t say the “hottest century” or the “hottest millennium”.