John Rankin, of Kelburn, asks :-

Is global warming scepticism justified? I have been told that there has been no warming for 17 years.

Philip Sutton, a physical oceanographer with NIWA, responded.

My first response is that scepticism is always justified. Science is about asking questions and thinking about how things work. That said, there are provisos. There are a number of reasons the climate debate is contentious. Primary among these is the importance of the issue and its ramifications in terms of money and lifestyle. A second factor is that while the internet has enabled people to research the topic, with no peer-review or quality control, the quality of information can be dubious. If people are genuinely interested, they need to check the sources and read the peer-reviewed publications that are the foundation stones of scientific discussion.

The ‘no warming for 17 years' assertion relies on 1998 being an exceptionally warm year - in fact 1998 is thought to be the 4th or 5th warmest year on record (with all the warmest years having occurred since 1998). The explanation is that climate varies on many time scales, and just because 1998 was particularly warm does not mean that the variability is not part of a continuous subtle warming. This argument would become much stronger if no change is seen in the next 10-20 years.

This question was passed to me, an oceanographer. Water has a very high ability to absorb heat, hence the ocean dominates the heat storage of the climate system. The oceans are responsible for about 93% of the global heat content, which determines whether the world is warming more accurately than surface air temperature.

I have just been involved in a study determining global ocean heat content changes since 2006, when an ocean measuring system of profiling floats called Argo ( reached global coverage. This study indicates that ocean heat content steadily increased between 2006 and 2014. The conclusion is that there is no sign of the so-called ‘warming hiatus’ in heat content. Of interest to New Zealand is that almost all of the changes have occurred south of 20°S. This is likely because of the large area of the southern hemisphere ocean and the special case of the Southern Ocean with uninterrupted flow and exposure to strong winds around Antarctica.

In summary, I think scepticism is good, but should be constructive and well-founded. I believe there has been warming since 1998, based on studies I have been involved with using ocean data and other studies I have read. I cannot speak for ‘climate scientists’ as a group, but the Intergovernmental Panel on Climate Change (IPCC) expresses the scientific consensus and its reports are available online at