Skeptical Science blog has a little widget that counts the heat the climate has accumulated since 1998 in terms of Hiroshima Atomic Bombs.

One the first uses of the Hiroshima bomb analogy was by skepticalscience.com stalwart Dana Nuccitelli, in the Guardian.

The rate of heat building up on Earth over the past decade is equivalent to detonating about 4 Hiroshima atomic bombs per second. Take a moment to visualize 4 atomic bomb detonations happening every single second.

But what does this mean in actual heat energy? I did a search, and found out the estimated heat generated by the Hiroshima bomb was about 63TJ, or terra joules, or 63 x 10¹² joules. A quick calculation reveals the widget actually uses 62TJ, so I will use that lower value. It is a huge number. The energy was sufficient to kill over 100,000 people, cause horrific injuries to many more, and destroying every building within a large radius of the blast site. Yet in the last 17 years the climate system has accumulated over two billion times that energy.

Most of that energy goes into the oceans, so I was curious to estimate the impact that phenomenal heat accumulation would have on the average temperature of the oceans. Specifically, how long would it take to heat the oceans by 1^oC.

The beauty of metric measurements is that weight and volume are combined all around the unit of water. I will ignore the slight differences due to the impurities of sea water for this exercise.

The metric unit of energy, a joule, is not quite so easy to relate to water. The old British thermal unit is better, being the quantity of energy sufficient to raise a pound of water through 1^oF. Knowing that 1lb=454g, 1.8^oF = 1^oC and 1btu ≈ 1055J, means that about 4.2 joules is the energy sufficient to raise 1 gram of water the one degree.

So the Hiroshima bomb had the energy to raise (62 x 10¹²)/4.2 ≈ 15 x 10¹² grams of water through one degree.

That is 15 x 10⁹ kilos (litres) of water, or 15 x 10⁶ tonnes (cubic metres) of water. That is the volume of a lake of 1 kilometre in area, with an average depth of 15 metres.

The largest lake in England is Lake Windermere, which has approximately a volume of 1 cubic kilometre of water, or 1 billion tonnes of water. (The biggest freshwater lake in the United Kingdom by volume is Loch Ness, with about 9 km³ of water.)

It would take the power of 67 Hiroshima bombs to heat Lake Windermere by 1 degree. Or the oceans are accumulating heat at a rate that would the temperature of this lake by one degree in 16.67 seconds.

Although Lake Windermere can look quite large when standing on its shoreline, it is tiny in relative to the Great Lakes, let alone the oceans of the world. With a total area of about 360,000,000 km², and an average depth of at least 3000 metres, the oceans have a volume of about 1,080,000,000 km³, or contain 108 x 10¹⁸ tonnes of water. If all the heat absorbed by the global climate system since 1998 went into the oceans, it would about 18 billion seconds to raise average ocean temperature by 1^oC. That is 5,000,000 hours or 208,600 days or 570 years.

Here I am slightly exaggerating the warming rate. The UNIPCC estimates that only 93% of the warming from extra heat absorbed by the climate system was absorbed by the oceans.

But have I got this wrong by a huge margin? The standard way of stating the warming rates – used by the UNIPCC – is in degrees centigrade per decade. This is the same metric that is used for average surface temperatures. Warming of one degree in 570 years becomes 0.0175°C/decade. In Chapter 3 of the UNIPCC AR5 Working Group 1 Report, Figure 3.3 (a) on page 263 is the following.

The ocean below about 1000 metres, or more than two-thirds of the water volume, is warming at a rate less than 0.0175°C/decade. This may be an overstatement. Below 2000 metres, average water temperature rise is around 0.005°C/decade, or 1^oC of temperature rise every 2000 years.

The energy of four Hiroshima bombs a second is trivial on a global scale. It causes an amount of temperature change that is barely measurable on a year-on-year basis.

There are two objectives that I believe Skeptical Science team try achieving with their little widget.

The first objective is to reverse people’s perception of reality. Nuclear explosions are clearly seen by everybody. You do not have to be an expert to detect it if you are within a thousand miles of the detonation. Set one off anywhere in the world, even deep underground, and sensitive seismic detectors will register the event from the other side of the globe. Rejection of the evidence of a blast can only be on the basis of clear bias or lying.

But trying to measure changes of thousands of a degree in the unimaginable vastness of the oceans, with changes in the currents and seasonal changes as well is not detectable with a single instrument, or even thousands of such instruments. It requires careful collation and aggregation of the data, with computer modelling filling in the gaps. Small biases in the modelling techniques, whether known or unknown, due to technical reasons or through desiring to get a particular result, will be more crucial than accuracy of the instruments. Even without these issues, there is the small matter of using ten years of good quality data, and longer periods of sparser and lower quality data, to determine underlying trends and the causes of it. Understanding of the nature of the data measurement issue puts the onus on anyone claiming the only possible answer to substantiate those claims.

The second objective is to replace a very tiny change in the very short period for which we have data, into a perception of a scientifically-validated catastrophic problem in the present. Whether it is a catastrophic problem relies on the projections of climate models.

It is easy to see why Skeptical Science needs this switch in the public perception of reality. True understanding of climate heat accumulation means awareness of the limits and the boundaries of our current knowledge. That requires a measure of humility and recognition of when existing knowledge is undermined. It is an inter-disciplinary subject that could result in a whole range of results of equal merit. It does not accord with their polarized vision of infallible enlightened scientists against a bunch of liars and ignoramuses who get nothing right.

Kevin Marshall