Tuesday, March 26, 2013

World's oceans are getting warmer, faster

When looking at how global temperatures have changed, it's easy to focus on the atmosphere. But as a new paper shows, we should be looking at the oceans too - and the deep ocean in particular. Over the last half century, new data shows the oceans have warmed substantially - accelerating in the last decade.

Since the start of the 20th century, land and sea surface temperatures have risen by about 0.76 degrees Celsius, mainly as a result of burning fossil fuels. But the heat that stays trapped in the atmosphere is only a small fraction of the sun's energy that hits the earth. Previous studies show about 90 per cent of the heat is absorbed by oceans.

The new study, just published in the journal Geophysical Research Letters, shows just how much the oceans have warmed in the past 50 years as a result. Notably, it finds the rate of warming has accelerated since about 2000.

Observing the oceans

The scientists combined historical ocean temperature data collected in a number of different ways: remotely-sensed data from satellites, direct measurements made by instruments deployed from ships and free-drifting vehicles known as ARGO floats.

The team fed the measurements - spanning the period 1958 through to 2009 - into an ocean and atmospheric circulation model from the European Centre for Medium-Range Weather Forecasts (ECMWF). The model simulates the historical observations and produces an estimate of warming at different depths in the ocean every 10 days.

Long term warming

The purple lines in the graph show how the heat content of the whole ocean has changed over the past five decades. The blue lines represent the top 700 m and the grey lines are just top 300 m.

Each of the five lines in each set represents a single model run, each with slight differences in the starting conditions or in details of the model. This is one way scientists account for uncertainty in the climate system or in the measurements themselves.

The data show a general warming trend in the oceans over the last five decades - but there are a couple of important features to notice.

Short, sharp cooling events

There are a few times in the last half century when the whole ocean has lost a lot of heat to the atmosphere quite suddenly after a major volcano eruption, such as Pinatubo in 1992.

A natural periodic fluctuation in the climate known as the El Nino Southern Oscillation (ENSO) also affects the amount of heat stored in the ocean. In El Nino years, notably 1997 to 1998, the ocean releases a lot of that heat into the atmosphere - causing atmospheric temperatures to warm.

Previous studies that have looked at how the ocean's uptake of heat has changed over time have only been able to "hint at" the short, sharp cooling events that have punctuated the long term warming record, co-author Professor Kevin Trenberth told Carbon Brief.

Faster warming

Probably the most important feature for analysing recent global temperature is that the rate of ocean warming has accelerated sharply since about 2000 - as the rise in the purple line in the graph shows. But the warming isn't evenly distributed throughout the ocean. Warming in the top 300 mm of the ocean - shown in grey - has slowed slightly since about 2000.

Only the deeper ocean shows accelerated warming, at a rate unprecedented in the last 50 years. This is pretty clear in the blue line - which is the top 700 m - but the pattern is strongest in the model data for the whole ocean, which extends to below 5000 m.

The scientists calculated that warming of the ocean below about 700 m now accounts for about 30 per cent of the total heat entering the oceans. The new paper also suggests a reason why more heat is being transported out of the surface ocean. Trenberth tells us:

"The cause of the change is the change in winds, especially in the Pacific Ocean where the subtropical trade winds have become noticeably stronger, thereby increasing the subtropical overturning in the ocean and providing a mechanism for heat to be carried down into the ocean."

Missing heat

The new study helps shed light on an issue known as the ' missing heat' problem. Scientists can calculate how much the planet should be warming based on the amount of solar radiation hitting earth and the level of greenhouse gases in the atmosphere. But since 2004, measurements of heat uptake by the land, the surface oceans, the atmosphere and melting ice have fallen short of the expected warming.

Together with the fact that global atmospheric temperatures have risen slower over the past decade compared to previous decades, this has led to claims by some that global warming has paused or even stopped. The new study doesn't completely close the gap - possibly because of some errors remaining in the measurements, Trenberth tells us. But it does show where a lot of the 'missing heat' is going - into the deep ocean.

Trenberth tells us:

"[The new study] also means that the current hiatus in surface warming is a transient and global warming has not gone away."

Global warming isn't all about atmospheric temperatures. The new study highlights the importance of looking at the surface of the ocean right down to the deepest depths when trying to understand how sensitive the atmosphere is to increasing greenhouse gases - a concept known as the climate sensitivity. The role of the oceans in climate change - particularly below 700 m - can't be overlooked.

Source: The Carbon Brief

Image courtesy of E Photos via Flickr (CC BY 2.0)