WeatherWatch weather analyst Philip Duncan's blogs on climate change have attracted a lot of reader interest. He took readers' 10 most commonly asked questions and put them to Dr James Renwick, Principal Scientist, Climate Variability & Change at Niwa.

Philip Duncan: 1) It feels like summers in New Zealand aren't as hot as they used to be - it doesn't feel like NZ is getting hotter at all?

Dr Renwick:

I hear that comment quite a lot, and I think a lot of it is psychological. My perception is that when you're young you spend a lot of time outdoors in the summer...the older we get the more time we spend indoors, in the office, less holidays etc. So our perception is that summers used to be hotter, but I can assure you that the data show it is definitely warming up and has done so over the last century.

2) If the world is heating up, why are places like USA and UK seeing record cold and snow?

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That's the difference between global change and regional change. The USA and UK have had a very cold winter, but other parts of the northern hemisphere, such as Greenland, Alaska and the Arctic Ocean have been much warmer than average over the past few weeks, but this didn't make the headlines. We need to be careful with comparing a local region to what's happening across the whole globe.

3) Why was Global Warming replaced with the term "Climate Change"?

That's a really interesting question and I don't believe it ever was changed. My perception is that Climate Change has always been used in the scientific community, however the term "Global Warming" was something perhaps used more by the media and then the term stuck. There's a lot more to climate change than just warming - that's why all the scientists I know use and have always used the term "Climate Change". I don't think there has ever been an "official" replacement.

4) Last decade was NZ's warmest decade on record - but wasn't the increase within the margin of error or at the very least, a tiny change?

There are two ways of looking at that. One of the records we used was based on seven climate stations which have data for well over 100 years. The difference in the averages (between the 2000s and the 1980s) from those stations was indeed very small and within margin of error. However it's important to note that four decades in a row have been significantly warmer than those before it. There are other records that can be used, such as the 11 reliable climate sites we described on our website last year, and in that data set the 2000s and the 1980s (next warmest decade) were more than one 10th of a degree different, which is significant. So the last 10 years were a bit warmer, and the last few decades have been a lot warmer than all the previous decades in the record, which shows an overall warming trend. If it was natural variability then you would expect a recent decade to have been cooler, like it was in the 1920s, say - but we aren't seeing any decades dropping back to those sorts of levels - and it's very unlikely going into the future that any will be that much cooler.

5) If the world is getting hotter, how come 2009 was cooler than average in NZ?

This is partly the same as question 2. It's important to remember that New Zealand covers a small fraction of the globe. Climate change doesn't mean every year will be warmer in every country. It also doesn't mean every year will be warmer globally. There are always ups and downs but the trend is upwards. For instance, last century the eastern US actually cooled for several decades while the globe overall warmed up significantly. In the last 25 years however, that local cooling has reversed, as the globe has continued to warm. So, the overall trend is upwards, but even then we do see some cooling regions and some cooler periods. There are patches of the globe, sometimes quite large patches, which can go against the overall trend for a while - but that misses the point...if you're thinking about global change you have to look at the whole globe.

6)Weather forecasters can't even predict the weather 2 weeks out, how can climate scientists predict 10 or 50 years out?

Well that's confusing the weather with the climate. Its true you can't predict the exact daily sequence of the weather more than a week or two out. But we (climate scientists) can say, with quite a lot of certainty, that July is going to be cooler than January in Auckland because seasonal change in the climate is predictable...and changes in the average climate over decades are also quite predictable. We're not in the business of saying what the weather will be like in Jan 2050, but we can predict average conditions several decades out on the basis of greenhouse gas increase. Here are a couple of analogies that might help explain my point:

Analogy1: Imagine you're out in the harbour on your boat. Predicting the weather is a bit like predicting the ripples of the waves on the sea caused by the wind. Predicting the climate is more like predicting the tides.

Analogy 2: Nobody can predict exactly when you and I will die, but insurance companies make a lot of money from knowing what the average death rate is - this average can't be applied to any individual. Predicting the climate is like using those life expectancy tables...we can predict the averages and the overall statistics with a fair degree of accuracy. Predicting the weather is more like tracking an individual person...certainly more variable.

7) Is 30 years of weather data long enough to use as a "guide" for predicting the future?

The four warmest decades where the last four decades...and some have interpreted that to mean we didn't have data from before that. Actually, we have good records from a lot of stations in NZ from the last 70 or 80 years and some back well over 100 years. Globally, scientists use ice cores, tree rings, and other records to estimate climate over several hundred or thousand years. 30 years is certainly not long enough but no one is actually using just 30 years of data.

7B: You just mentioned ice cores - but haven't they showed big warmings in the past?

Yes you're right, and that tells us the climate is sensitive. ...it just happens that it has been pretty stable while human society has been around. There are certainly natural things that cause fluctuations, such as changes in the Earth's orbit that drive the ice ages, but it's also clear that human activity - CO2, Greenhouse gas release affects the climate as well...in fact, basic physics shows that today's greenhouse gas release is a much faster way to heat the planet than the slow natural warming process that ends an ice age. I often hear this argument: Because there are natural causes of variation then the concept of human-caused variations is impossible - i.e. there's been natural cause in the past so then that rules out a human cause now. That argument just doesn't make sense. Natural influences have caused the climate to change quite a lot in the past and that should give us concern...it shows that the climate is variable, and vulnerable.

8)Are scientists scared to speak out about what they really believe for fear of being alarmist or not "going with the consensus"?

Well no (laughing)...no not at all. Scientists are in the business they're in because they want to find out what's actually happening with the natural world. I don't know one scientist who is scared to speak out about what they believe...and believe is an interesting word. Science is about observing the natural world and building understanding on those observations, it's not about belief. Scientists publish their results openly, there's no fear of speaking out at all. Going with the "consensus" is an illusion too. For instance, the IPCC is a review process - it summarises what thousands of scientists all over the world have observed, or modelled, or deduced - it doesn't dictate to them, just summarises. It turns out 99.9 per cent of work reported does indeed form a consensus... that's a reflection of how things are, of what the real world looks like - it's a very clear picture.

9) Why does it seem that climate change is so doom and gloom? I'm burnt by warnings of things like SARS, Y2K, Bird Flu, Swine Flu etc how can I trust the experts on this one?

There isn't an easy answer to that...there have been a lot of things in the media that haven't turned out to be as important or dangerous as we are led to believe...and if you're not an expert in those fields then what do you believe? What I can say is that with climate change there is an incredible weight of evidence that shows that climate change is happening...it is definitely a problem. Almost every scientific paper out there supports this view. The IPCC process is designed to help non-experts understand the problem.

There are a lot of dire possibilities with future climate change...all the scientist I know are very concerned, and have a sense of urgency about taking action. To help convey that sense or urgency to the public, we sometimes do focus on the biggest issues or risks. It's important to note though that it's not all doom and gloom...there will be some winners, at least in the short term....perhaps more grass or grape growth in colder parts of New Zealand...but unfortunately there will be more losers than winners for the globe, and more at risk as times goes on.

10) Copenhagen seemed like a complete waste of money...are politics helping or confusing your cause?

Science is about evidence and understanding the natural world, so scientists are not in the business of politics. But there has to be a political process to deal with this problem, one that's informed by the science. There is no alternative to dealing with this, and individual countries need to work together. Getting that cooperation going can seem slow and confusing, and a bit of a time wasting process.

From my point of view it would be great if the political process was more efficient and faster. Copenhagen was a bit disappointing...but given human nature, I guess the world community has to go through these stages before we really get somewhere. I was just reading a report on Copenhagen (from the Business Council for Sustainable Development)...and the author said it reminded him of going to a dance as a teenager...it takes a long time to get couples up on the dance floor... but eventually someone gets up and dances, and then everyone wants to dance. That was Copenhagen - things didn't really get going there, but we hope the big players will be ready to dance soon.

In the scientific community there is a great deal of concern and a sense of urgency - that we have to do something now. This hasn't quite gotten through to the international political world. We aren't taking it seriously enough yet...and there isn't a lot of time left to get on top of things.

Click here to watch James Renwick discussing the economic benefits of being able to predict long term climate, and how to approach climate change.