News Arctic: Four Hiroshima bombs a second: how we imagine climate change

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Saturday, 17 August 2013

Four Hiroshima bombs a second: how we imagine climate change

Hiroshima bomb - from: Wikipedia image

Where does the excess heat go that is trapped in our atmosphere by greenhouse gases every day?

The title of this post is a hat-tip to David Holmes, Monash University, Australia, who recently published an article with that title at The Conversation, discussing that the daily excess heat absorbed by Earth equals the heat released by well over four Hiroshima bombs every seconds.

It’s actually well over four Hiroshima bombs every second, given that there are 86,400 seconds in a day and based on James Hansen calculations (at a Feb 29, 2013, TED presentation) that the current imbalance of 0.6 watts/square meter (which does not include the energy already used to cause the current warming of 0.8°C) was equivalent to exploding 400,000 Hiroshima atomic bombs every day, 365 days per year.

As illustrated by the graph below, most of this excess heat is absorbed by oceans and ice. Some of the heat is consumed by the process of melting ice into water, but most heat ends up warming up the oceans.

Graph by Sceptical Science

An earlier post (September 2012, added underneath) described the study by Nuccitelli et al. that measures heat going into the oceans in Joules and, as discussed above, measuring excess heat in terms of heat released by nuclear bombs might give more meaning to what is going on.

Where does the extra heat go?

Global warming is causing Earth to heat up. As shown on the image below, by Nuccitelli et al., most heat goes into the oceans. A substantial amount of heat also goes into the melting of ice.

Warming of water in the Arctic Ocean

Global warming is heating up the oceans big time. As the image below shows, the global ocean heat content has been rising for many years.

White arrows mark ice drift directions. Red arrows mark
the transport path of warm Atlantic water entering the
Arctic where it submerges under the cold, ice-covered
surface layer. Robert Spielhagen (IFM-GEOMAR, Kiel)

The Arctic is affected in particular by the Thermohaline Circulation.

Water flowing into the Arctic Ocean from the Atlantic Ocean is about 2°C warmer today than it has been for at least 2,000 years, according to a study published in Science. The current of warm water lies 50 metres below the surface, and can reach 6°C in summer — warm compared to Arctic surface waters, which can be -2°C.

At the same time, cold water and sea ice are driven out of the Arctic Ocean, along the edges of Greenland. The net result is a marked increase in the temperature of the water in the Arctic Ocean, especially the top layer of the water which causes the sea ice to melt.

The Arctic radiates comparatively less heat into space

Furthermore, cold layers of air close to the surface of the Arctic Ocean make it difficult for infrared radiation to go out to space, according to a study published in Science. These layers do warm up, but warming of these layers is directed downwards, thus amplifying warming in the Arctic.

Surface air temperatures in the Arctic are rising rapidly

Anomalies for surface air temperatures are higher in the Arctic than anywhere else on Earth. The increase in temperature anomalies appears to be an exponential rise. This is caused not only by the above-described points, but also by feedback effects as further described below.

How much will temperatures rise?

In the above graph, rising temperatures are compared to the global average for the period 1951-1980, which is typically used as a base period by NASA in temperature change analysis. The background behind this is that the U.S. National Weather Service uses a three-decade period to define “normal” or average temperature. The NASA Goddard Institude for Space Studies (GISS) analysis of temperature anomalies began around 1980, so the most recent 30 years at the time was 1951-1980.

The study ‘Climate Impact of Increasing Atmospheric Carbon Dioxide’, by NASA scientists led by James Hansen, describes those early efforts and was published in Science back in 1981. The image below is from the paper, showing that much of the extra heat trapped by carbon dioxide released by people in the atmosphere ends up in oceans.

The paper discusses how many years it can take for oceans to warm up, and the role of feedbacks in that process. The paper notes that a surface albedo change over land areas of 5% (equivalent to a 1.5% global change), would affect global temperature by 1.3°C, adding that paleclimatic evidence suggests that surface warming at high latitudes will be two to five times the global mean warming, due to snow/ice albedo feedback and greater atmospheric stability, which magnifies the warming of near-surface layers.

Feedbacks further accelerate warming in the Arctic

Feedbacks are described in more detail in posts such as Diagram of Doom (image below) and Changes to Polar Vortex affect mile-deep ocean circulation patterns.

Diagram of Doom

One such feedback is albedo change — retreat of Arctic sea ice results in less sunlight being reflected back into space, as further discussed in Albedo Change in the Arctic. Loss of Arctic sea ice is effectively doubling mankind’s contribution to global warming. Increased absorption of the sun’s rays is the equivalent of about 20 years of additional CO2 being added by man, Professor Peter Wadhams said in a BBC article.

One of the most threatening feedbacks is release of methane that are held in the currently frozen seabed. As the seabed warms up, it starts to release methane in what can be rather abrupt ways. Due to methane's high global warming potential and low levels of hydroxyl in the Arctic, this threatens to further accelerate local warming and trigger further methane releases, in a vicious spiral of runaway global warming.