So here’s the chart of HadCRUT data for the years 1980-2016. Our period of interest starts after the spike in the middle of the chart.
There are two distinct phases post 1998. First, a lack of overall increase in temperature between 1998 and 2014 (the infamous “hiatus”). And then a record breaking strong increase in temperature in the last two years.
The previous post finished just after the record year of 1998. There were prophets of climate doom, but also deniers who said the earth wasn’t warming at all. The chart since then shows two things; firstly the hiatus gave a lot of support to deniers, or those who argues for some kind of natural variation. It also left a number of people who made the more alarmist claims looking slightly stupid. The sharp increase in the last two years however demonstrates that warming is clearly happening. Not shown on the charts is the fact that now all measurements including satellite measures are in agreement.
We know as naturalists that there have been significant population movements in the last 20 years. Most inspect species are moving north, or moving higher. There is a long list of insect and bird species that have expanded their range from Europe to the UK, and conversely we see a disappearance of many bird species from the southern parts of their range even where there has been no change in habitat. Spring migration is happening on average 2 weeks earlier than it was thirty years ago. It is hard to avoid the conclusion that if all our measurements show global temperature increasing, and all our flying creatures are behaving as if the earth is warming, then the earth is warming. Anyone who doubts this warming needs to explain not just why all the different methods of measurement all show this, but why the natural world has responded as if the world is warming.
Having established that warming is happening, the next questions – how fast and what happens next – are much harder to call. Scientific theories do two things: allow us to understand the past, and allow us to predict the future (even if only in a conditional if we do this, then that will happen). Climate science is reasonable at explaining the past, but is currently poor at predicting the future. The chart of the temperature anomaly above is a good illustration of this. No-one predicted the hiatus, but efforts are now underway to explain why we have this choppy behaviour in temperature behaviour. The emerging explanation centres round the importance of the oceans in global warming and the role of El Nino.
Firstly some quick calculations.
Heat capacity of the ocean = 1.35 x 10^21 Kg x 3.993 KJ/Kg/C (specific heat capacity of salt water) = 5.4 x 10^21 KJ/C
Heat Capacity of the atmosphere = 5.1 x 10^18Kg x 1 KJ/Kg/C = 5.1 x 10^18 KJ/C
So the sea holds roughly 1000x more heat than the atmosphere (if they are at the same temperature). Hence over a long period the oceans play a key role in global temperatures. The temperature of the atmosphere can not, over a long period, diverge too far from its historic relationship. Thermal energy is transferred between the two. The mechanism that seems to have a significant role in how and when this is done is the global weather phenomenon of El Nino and its twin La Nina.
El Nino and La Nina.
El Niño and La Niña are opposite phases of an oscillatory climate cycle in the Pacific ocean, and have been known about since the 1600s.
No-one really understands the mechanics of el Nino, or can predict when it will happen and how strong it will be [1,2]. Under normal conditions, trade winds blow west from the equator keeping warm surface water off Asia. In El Nino years these winds do not blow as hard and the warm water flows to the American west coast bringing rain with it. La Nina is the opposite; the winds blow strongly restricting the flow of warm water. The consequences are far-reaching around the globe, and one consequence we are beginning to understand is that El Nino releases heat from the ocean into the atmosphere.
To illustrate this I’ve taken a table of El nino/La nina years  divided by strength and calculated the average global temperature anomaly in the subsequent year (ie 1998 anomaly for the el Nino in 1997-98). This is shown (poorly – mac Excel is horrible) below
The labels on the right go from -S (Strong La Nina) to <> ("normal") to +VS (Very Strong el Nino)
This shows generally that the stronger El Nino, the higher the temperature anomaly.
Current thinking then is that during the Hiatus years 1998-2013 the world was still absorbing thermal energy, but this was being stored by the oceans. The El Nino of 2015-16 which was a very strong one has released a lot of heat and so seen a significant sharp increase in temperature. My prediction is that in the next few years we may see a hiatus established for a decade or so at this new higher temperature, and then some years down the line the process repeats and the temperature moves up sharply. But that’s just my view.
Next I will look at the opinions around why the world is heating starting with greenhouse gases.