|I get up in the morning and everything looks fine. The Earth is still spinning on its axis, still wobbling around it’s axis, and still encircling, or rather enellipsoiding, the Sun. Birds tweet, the grass rises, and there’s the usual random selection of weather.
But, almost invisible, there’s a climate emergency, an ongoing and grinding crisis happening right here, right now, demanding my attention.
|Despite what some would have me believe, climate change is not a low-level, marginal effect. Although it seems at the moment that we have plenty of time to adapt to changing circumstances, the problems are mounting up.
You see, climate change is not happening in a steady, measured manner. There are some climate indicators that are not only rising, but accelerating. The pace of change is racing ahead. Climate change is already having a significant effect, and as change speeds up, these effects will become dangerous.
Some people are not aware of these dangers in the Earth’s climate system, but it doesn’t make them any less real or any less serious. It’s time that people in general had better access to the facts.
ACCELERATION OF THE AMOUNT OF CARBON DIOXIDE IN THE AIR
Of all the greenhouse gases, carbon dioxide is the most serious. It persists in the atmosphere – it doesn’t rain out of the sky like water vapour, and it doesn’t dissociate into less powerful greenhouse gases like methane does. The rate of change of the level of carbon dioxide in the Earth’s atmosphere is increasing – it is accelerating. The chart at the top of this page is perhaps the simplest way to show this. Here’s another chart showing the raw data :-
The amount of carbon dioxide being added to the atmosphere above our heads has gone from a range of 1 to 2 parts per million per year in 1960, to 2 to 3 parts per million per year now. That’s acceleration – a rise in the rate of increase.
WHAT EFFECT IS THIS HAVING ON GLOBAL TEMPERATURES ?
The global temperature record is complex – the average over one region is often not the same as the average in another region. The temperatures of land and ocean change in different ways. Plus, there are quite large swings year to year. And in addition, the profile of temperature change is different if you look at different parts of the Earth system – for example in the deep oceans, near the ocean surface, the land close to sea level where most of us live, and the high-up atmosphere.
Over short periods of time, the largest effects on Earth near-surface temperatures are the cooling effects from the material and gases thrown up into the atmosphere from volcanic eruptions, and the big cyclical ocean pattern known as ENSO – the El Nino Southern Oscillation. Here’s how a research paper in 2011 summarised the effect of ENSO, showing surface heating and cooling :-
There are several mechanisms being used to monitor average global temperature at the most important area for us – around about sea level in the land and shallow oceans, and in the lower part of the atmosphere. In 2011, a research paper took at look at all these data sets and how they could be smoothed to remove the influences of volcanic eruptions and the ENSO. Here is what Foster and Rahmstorf found for the underlying global warming caused by greenhouse gases :-
All the data show the same rate of global warming more or less, which is important because the data was measured in different ways. However, it doesn’t show an acceleration in global warming – there is a constant rate of heating. Each year the Earth warms by the same kind of amount.
CARBON DIOXIDE IS NOT THE ONLY GREENHOUSE GAS
Even though the rate of carbon dioxide being added to the air is increasingly rising – this is not making global warming speed up, it seems. Why is that ? Well the short answer is that carbon dioxide is not the only greenhouse gas, and that besides greenhouse gases, there are other influences on the heating and cooling of the Earth’s surface. Here’s a more accurate picture of the sum total of the influences at work – what Professor James Hansen calls “the forcings” :-
The big dips you can see are due to volcanic eruptions, as can be seen from Figure 1 in a 2011 paper from Hansen and his colleagues.
THE BALANCE OF HEATING AND COOLING
The measure of radiative forcing is a figure to indicate how much extra energy from the Sun is being trapped in the Earth system and converted into heat. In Hansen’s team’s analysis, the radiative forcings do not include the temporary heating and cooling effects of the ENSO cycle. When these are included in the analysis, interestingly, there are periods where the warming and cooling effects of all the influences on the Earth’s temperature, including solar activity, volcanic eruptions, and the greenhouse gases and airborne particles coming from humankind’s activities, can be expected to balance out. A 2011 research paper by Robert K. Kaufmann and colleagues says this :-
“…anthropogenic activities that warm and cool the planet largely cancel after 1998, which allows natural variables to play a more significant role. The 1998 – 2008 hiatus is not the first period in the instrumental temperature record when the effects of anthropogenic changes in greenhouse gases and sulfur emissions on radiative forcing largely cancel. In-sample simulations indicate that temperature does not rise between the 1940’s and 1970’s because the cooling effects of sulfur emissions rise slightly faster than the warming effect of greenhouse gases. The post 1970 period of warming, which constitutes a significant portion of the increase in global surface temperature since the mid 20th century, is driven by efforts to reduce air pollution in general and acid deposition in particular, which cause sulfur emissions to decline while the concentration of greenhouse gases continues to rise…” [Kaufmann et al. (2011)]
RADIATIVE FORCING CONTINUES TO RISE
Yet, although the heating of the lower part of the atmosphere was more or less zero during the period between 1998 and 2008, the net radiative forcing was still positive, and higher than in previous decades. Another period where the net warming of the Earth was negligible was in the decade 1940 to 1950. Comparing the decade 1940 – 1950 with the decade 2000 – 2010, we can see how the net radiative forcing has increased, a trend that is continuing :-
Most of the extra heat in the Earth system as a result of global warming ends up in the oceans :-
As the oceans get progressively hotter, seawater heats up and expands, and added to this, as glaciers and land-based ice sheets melt, we can expect to see sea levels rise. We cannot do a direct comparison of the net radiative forcing and sea level rise for any decade, because the ocean takes a long time to warm up, and glaciers and ice sheets take a while to melt down. It’s rather like waiting for a pot of soup to boil on a wood fire, or for an icicle on your roof to melt when the sun comes out. It can be several decades before a heating force can have an effect on sea levels. In the decades 1940 – 1950 and 2000 – 2010, even while heating and cooling were roughly balanced in the upper ocean waters and the near-to-the-ground level in the air, sea levels were continuing to rise :-
We can see the overall impact of radiative forcing by looking at the whole sea level record :-
There is a great deal of variation in sea level, owing to a number of changes in ocean circulation, and other effects, but it can be seen that in the period of the Church and White (2011) data, there has been an acceleration in sea level rise – an increase in the increase. Not only that, but the moving 5 year average has a steeper rise between 1950 and 2010 compared to the whole of the period 1885 – 2010 :-
(Note : 1950 was chosen as the cut-off year, because this is the year in which greenhouse gases in the atmosphere started to accumulate more than previously – see Figure 18 in Hansen et al. (2011).)
BUT SEA LEVEL RISE ACCELERATION LOOKS NEGLIGIBLE
One could think that this acceleration in sea level rise is not very significant, but the key point is that it is indicative of the extra heat being stored up in the oceans, making the water expand. The vast majority of the heat from global warming is stored in the oceans, and the increase in ocean heat content is a major factor in maintaining and promoting the increase in air temperatures :-
“…We apply a statistical method called cointegration analysis to observed global sea level and surface air temperature, capable of handling […] peculiarities. We find a relationship between sea level and temperature and find that temperature causally depends on the sea level, which can be understood as a consequence of the large heat capacity of the ocean…” [Schmith et al. (2012)]
And looking at ocean heat content is where we turn next…