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Author: Karl Braganza
Date: 14/06/2011

CLEARING UP THE CLIMATE DEBATE: Bureau of Meteorology scientist Karl Braganza explains why we know the climate is changing, and what’s causing it.

In public discussions of climate change, the full range and weight of evidence underpinning the current science can be difficult to find.

A good example of this is the role of observations of the climate system over the past one hundred years or more.

In the current public discourse, the focus has been mostly on changes in global mean temperature.

The greenhouse effect is fundamental science

It would be easy to form the opinion that everything we know about climate change is based upon the observed rise in global temperatures and observed increase in carbon dioxide emissions since the industrial revolution.

In other words, one could have the mistaken impression that the entirety of climate science is based upon a single correlation study.

In reality, the correlation between global mean temperature and carbon dioxide over the 20th century forms an important, but very small part of the evidence for a human role in climate change.

Our assessment of the future risk from the continued build up of greenhouse gases in the atmosphere is even less informed by 20th century changes in global mean temperature.

For example, our understanding of the greenhouse effect – the link between greenhouse gas concentrations and global surface air temperature – is based primarily on our fundamental understanding of mathematics, physics, astronomy and chemistry.

Much of this science is textbook material that is at least a century old and does not rely on the recent climate record.

For example, it is a scientific fact that Venus, the planet most similar to Earth in our solar system, experiences surface temperatures of nearly 500 degrees Celsius due to its atmosphere being heavily laden with greenhouse gases.

Back on Earth, that fundamental understanding of the physics of radiation, combined with our understanding of climate change from the geological record, clearly demonstrates that increasing greenhouse gas concentrations will inevitably drive global warming.

Dusting for climate fingerprints

The observations we have taken since the start of 20th century have confirmed our fundamental understanding of the climate system.

While the climate system is very complex, observations have shown that our formulation of the physics of the atmosphere and oceans is largely correct, and ever improving.

Most importantly, the observations have confirmed that human activities, in particular a 40% increase in atmospheric carbon dioxide concentrations since the late 19th century, have had a discernible and significant impact on the climate system already.

In the field known as detection and attribution of climate change, scientists use indicators known as fingerprints of climate change.

These fingerprints show the entire climate system has changed in ways that are consistent with increasing greenhouse gases and an enhanced greenhouse effect. They also show that recent, long term changes are inconsistent with a range of natural causes.

Is it getting hot in here?

A warming world is obviously the most profound piece of evidence.

Here in Australia, the decade ending in 2010 has easily been the warmest since record keeping began, and continues a trend of each decade being warmer than the previous, that extends back 70 years.

Globally, significant warming and other changes have been observed across a range of different indicators and through a number of different recording instruments, and a consistent picture has now emerged.

Scientists have observed increases in continental temperatures and increases in the temperature of the lower atmosphere.

In the oceans, we have seen increases in sea-surface temperatures as well as increases in deep-ocean heat content. That increased heat has expanded the volume of the oceans and has been recorded as a rise in sea-level.

Scientists have also observed decreases in sea-ice, a general retreat of glaciers and decreases in snow cover. Changes in atmospheric pressure and rainfall have also occurred in patterns that we would expect due to increased greenhouse gases.

There is also emerging evidence that some, though not all, types of extreme weather have become more frequent around the planet. These changes are again consistent with our expectations for increasing atmospheric carbon dioxide.

Patterns of temperature change that are uniquely associated with the enhanced greenhouse effect, and which have been observed in the real world include:

  • greater warming in polar regions than tropical regions
  • greater warming over the continents than the oceans
  • greater warming of night time temperatures than daytime temperatures
  • greater warming in winter compared with summer
  • a pattern of cooling in the high atmosphere (stratosphere) with simultaneous warming in the lower atmosphere (troposphere).

How do we know it’s us?

By way of brief explanation, if the warming over the 20th century were due to some deep ocean process, we would not expect to see continents warming more rapidly than the oceans, or the oceans warming from the top down.

For increases in solar radiation, we would expect to see warming of the stratosphere rather than the observed cooling trend.

Similarly, greater global warming at night and during winter is more typical of increased greenhouse gases, rather than an increase in solar radiation.

There is a range of other observations that show the enhanced greenhouse effect is real.

The additional carbon dioxide in the atmosphere has been identified through its isotopic signature as being fossil fuel in origin.

The increased carbon dioxide absorbed by the oceans is being recorded as a measured decrease in ocean alkalinity.

Satellite measurements of outgoing long-wave radiation from the planet reveal increased absorption of energy in the spectral bands corresponding to carbon dioxide, exactly as expected from fundamental physics.

It is important to remember that the enhanced greenhouse effect is not the only factor acting on the climate system.

In the short term, the influence of greenhouse gases can be obscured by other competing forces.

These include other anthropogenic factors such as increased industrial aerosols and ozone depletion, as well as natural changes in solar radiation and volcanic aerosols, and the cycle of El Niño and La Niña events.

By choosing a range of indicators, by averaging over decades rather than years, and by looking at the pattern of change through the entire climate system, scientists are able to clearly discern the fingerprint of human-induced change.

Case closed

The climate of Earth is now a closely monitored thing; from instruments in space, in the deep ocean, in the atmosphere and across the surface of both land and sea.

It’s now practically certain that increasing greenhouse gases have already warmed the climate system.

That continued rapid increases in greenhouse gases will cause rapid future warming is irrefutable.

Read the whole series at The Conversation

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