Two issues ago we ran a provocative Viewpoint article by Chris de Freitas, of Auckland University, which questioned the currently accepted notion that global warming caused by human activity is abruptly changing Earth’s climate. We invited this Viewpoint to highlight the fact that, despite regular media reports attributing a great range of phenomena to anthropogenic (i.e. human-induced) global warming, there is still ongoing scientific debate on the issue—a matter which is rarely reported in the popular media.

That climate change triggered by human agency will lead to dire future consequences—a view promoted strongly by the United Nations Intergovernmental Panel on Climate Change (IPCC)­ has attained mantra status in some sectors, particularly with lobby groups and policy makers. The Kyoto Protocols are framed by the authority of this dominant theory. Our government has enacted, and signalled for the future, legislative changes to meet various commitments prescribed by New Zealand’s agreement to these protocols.

Three National Institute of Water and Atmospheric Research (NIWA) scientists, David Wratt, Brett Mullan and Dave Lowe, who have been engaged in climate research for 20 years and who contributed to the 2001 IPCC report, recently sent us a rebuttal of de Freitas’s Viewpoint. The position of the three NIWA scientists is that of the majority of the scientific establishment. The full text of both the de Freitas Viewpoint and the NIWA rebuttal can be viewed on our website (www.nzgeographic.co.nz/ viewpoint). Readers are encouraged to examine the arguments and form their own conclusions.

Here we merely summarise the NIWA response. To de Freitas’s claim that atmospheric carbon dioxide levels are being stabilised by increased plant growth and other feedback mechanisms, Wratt, Mullan and Lowe present a graph depicting steadily rising carbon dioxide levels measured at Baring Head between 1971 and 2002, and claim that atmospheric levels of the gas are increasing steadily. Worldwide surface temperature rises are real, they say, and not due to urban effects, as de Freitas argued. They are a little ambivalent about whether satellite temperature records (as opposed to Earth-based measurements) show no warming trend, as de Freitas strongly claimed.

While de Freitas put the view that any warming trend of the past 50 years is inconsequential once climate fluctuations over the last 1000 years are considered, Wratt and his colleagues see things very differently. According to their sources, the past 50 years have seen warming on a scale unparalleled in the preceding millennium.

Answering the point that climate change over the past century may have been driven by changes in solar output, the NIWA team quote the IPCC, which concedes there may be something to the solar argument but still considers that greenhouse-gas emissions are five to eight times as significant.

Is the sea level rising due to global warming? De Freitas noted it has been rising since the end of the last ice age. The NIWA group doesn’t directly disagree, but suggests global warming will cause thermal expansion of the ocean, producing rising sea levels for centuries to come.

Not surprisingly, the NIWA scientists disagree with de Freitas’s concluding comment that global warming will be unlikely to exceed 1º C over the next century. They state, “The 2001 IPCC Working Group I report, with its projection of a globally-averaged temperature increase of 1.4 to 5.8° C from 1990 to 2100, is a truly balanced assessment, drawing on input from a large number of experts and reviewers.”

What are we to make of these conflicting views? It is important to realise that the collection and interpretation of scientific data is often difficult and complex. Consider the question of whether satellite temperature measurements of the atmosphere show a trend over time. The question is simple. But in reality a number of satellites have probably collected such data—some French, some American, some Russian, perhaps Japanese. They have probably been in different orbits in different years over a 30-year span, measuring with different instruments at different times of the day or night, at different levels in the atmosphere. How accurately calibrated were the instruments? Did orbital decay affect the measurements? How comparable was the data they produced? How meaningful is it to average that data over the whole world—for a decade—to come up with, say, a 0.1º temperature increase? Scientists can have the best intentions and still produce different sets of data that, in the context of the climate debate, may suggest profoundly different outcomes when extrapolated far into the future.

In science, there are usually several different sets of ideas being evaluated on any topic at any time, and dissent is normal. In the field of climate change, the cautions and caveats of scientists collide with the bedrock conviction of environmental activism. There is pressure on science to come up with evidence quickly to avert supposed global catastrophe.

The difficulty with climate change is to distinguish between a run of aberrant weather and changing climate. In the same way that occasionally you can throw heads five or six times in a row, a run of hot years may occur entirely by chance. There is no substitute for time when struggling to distinguish between a string of coincidences and the start of a trend.

It is certain that climate has undergone great change in the past without any assistance from humans. And it is also certain that if humans could be removed from the Earth, and all anthropogenic effects undone, climate would continue to change. How, then, when we focus on a change in temperature over no more than 50 years, can we be sure that, first, the climate is changing and, second, that humans are responsible?

Some commentators seem to think that our climate must be preserved in its present balance at all costs, inferring that it was at some magical optimum a few moments before the start of the industrial revolution. This is utter nonsense. Earth’s climate is dynamic and cyclic. A mere 9,000 years ago, barely a blink in the cosmic scheme of things, our world was easing out of glaciation. Aucklanders of the time could have enjoyed a weekend stroll to Waiheke across the Hauraki Basin, thanks to dramatically lower sea levels. Around 40 glacial and interglacial cycles have occurred in the past two million years alone.

Seen in the light of this bigger picture, other variables—which humans have no hand in—must have played a dominant role in earlier eras. For instance, several recent studies suggest that galactic cosmic rays could be the main driver of global climate change. These rays are thought to ionise molecules in the atmosphere that then act as condensation nuclei, promoting cloud formation. Clouds reflect sunlight, leading to cooling. Every 140 million years, our solar system passes through the spiral arms of the Milky Way, home to a lot of star-formation activity and a source of cosmic rays. The authors of these studies note that Earth’s temperature oscillations over the past 500 million years have a period of about 135 million years and conclude that “cosmic ray flux alone can explain [approximately] 66 per cent of the total variance in the temperature data.” In this scenario, carbon dioxide has played a minor role at best.

And it is worth remembering that the main greenhouse gas is not carbon dioxide—it is water vapour, and humans have only an indirect impact on its concentration in the atmosphere. Warming caused by human activities promotes evaporation, and more water vapour in the air could translate into more clouds, which then reflect incoming sunlight back to space and lead to cooling of the atmosphere. Incorporating these sorts of possibilities into the climate models that form the basis for global-warming predictions is far from straight foward.

In a complex system, change in one small element can precipitate a gross change, and this is where the spectre of global warming seems most potent. As climates change, some species become less viable, and extinction may threaten.

But even extinction has its upside. After one major extinction event caused by a meteor strike and a massive climate shift, mammals moved from being lizard food to having a more promising outlook. A certain variant of simian became the dominant species. Global warming was responsible for the rise of Homo sapiens—we are an unforeseeable by-product of the greenhouse effect. Now we must consider to what extent we are responsible for another round of global warming.