Hiatus? What Hiatus..?

21 02 2014
Mark Cochrane

Mark Cochrane

Mark Cochrane, our resident Climate Scientist, has been ‘offline’ for quite some time, busy with work etc…..  Here is a little insight from Mark on the so-called hiatus in global warming.

Since about 1998, global surface temperatures have been largely flat. They are still creeping up but they are proceeding at nowhere near the rate of the previous decade or the average expected from global climate model ensembles. So what is going on? Is there something wrong with the whole idea of greenhouse warming?

We receive shortwave electromagnetic radiation (sunlight) from the sun. We’ve had satellites measuring this directly since the late 1970s. As anyone who has seen the famous “Blue Marble” images of the Earth knows, a portion of that sunlight bounces off with more reflection from clouds and snow than soil, vegetation and water. This is variable in space and time but comes out to roughly 30%. We’ve had satellites measuring this and longwave (heat) radiation too, so we know what went in and what came back out from the planet in terms of energy. The rest of that energy has got to be somewhere on Earth.

Of that remaining 70% of the solar energy reaching the earth but not reflected by natural substances, roughly half is deflected either directly (haze) or indirectly (cloud changes) or by aerosols (pollution). Another 20% has been screened by volcanic aerosols from a few large eruptions in the last half century (Agung, El Chichon, Pinatubo). Thankfully another 20% has been radiated back to space as the earth’s temperature has risen. This important negative feedback rises at the fourth power of the temperature. For those counting at home, that leaves 10% to cause all of the temperature changes that we commonly consider as ‘global warming’. Over 90% of that energy goes right into the oceans and so mankind is really whining about less than 1% of the energy changes that show up in the air and land temperature changes. (You can read Murphy et al 2009 for a fuller description).

But what has this got to do with the hiatus? Well, just think of it this way. Since the oceans take up over 90% of the incident radiating energy, just small changes in the way that energy is being distributed in the oceans will have significant impacts for the temperatures that we feel on the land. What could cause such a change? The ocean has a number of its own dynamic patterns that change the distribution of warmer and cooler waters. One well known pattern is the El Nino/La Nina (ENSO) phenomena (erratic 2-7 year changes). A less well know and longer period change is the Pacific Decadal Oscillation (PDO). A full understanding of how the phase changes occur is still lacking but the current thinking basically goes like this – The very strong El Nino of 1997-98 allowed warm water to spill back from the western Pacific and cross the oceans toward Peru.

The El Nino period exposed a lot of warm ocean water to the atmosphere and transferred a lot of heat to the atmosphere causing no end of weather havoc and cooling the ocean. The Pacific Decadal Oscillation (PDO) is somewhat similar but larger in extent than ENSO. Every 15-30 years it switches its own phases. In the warm phase from around 1920-1945 the Earth warmed rapidly. Then in the cold phase from 1946-1975, planetary temperatures stabilized in their last hiatus. From 1976-1998 we had another warming period. That’s about when the PDO went into another cold phase and the current ‘hiatus’ began.

This is what those ocean surface temperatures look like now:

Ocean_LandTemp

That whole center swath of the Pacific along the equator and the whole coast along North America is now anomalously low in temperature. This is a giant surface area of the planet and it acts as an energy sink soaking up solar heating. Strong Easterly winds blow the warming equatorial Pacific waters over toward Indonesia, piling warm water up there while drawing up cold deep water in the eastern and central Pacific. Temperature distributions start looking like this cross section which existed just before the 97-98 El Nino:

With lots of very warm (30 C) water being forced deep. This leads to increased transfer of energy into the deep oceans. Voila, the missing heat we notice at the surface of the land isn’t missing from the planet. It is just being put into deep storage. For a fuller description please see Trenberth and Fasullo 2013 and Tollefson 2014.

So while there may have been a pause in global surface warming, there has not been any hiatus in global warming as a whole. With the weather getting more erratic even at this relatively mild amount of warming, give a thought to what may come the next time the PDO shifts back to the warm phase and excess heat is released to the atmosphere.

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