Making climate change projections
Why can't scientists tell us exactly what will happen?
Humans are not responsible for all changes in the climate, but there has been a warming – by around 0.7º C globally – over the past 100 years. Much of that is likely to be due to greenhouse gases emitted into the atmosphere from human activity. Scientists expect concentrations of those gases to continue to rise well into the 21st century. Even though they can't predict precisely how these scenarios might play out, they nevertheless have to take them into account when they make climate projections.
To do that, they factor in a range of emissions scenarios. These are based on the various ways scientists think our economies, societies, population levels and energy use might look like in future. Different scenarios are based on whether greenhouse gases increase slightly, moderately or significantly.
When they made their climate projections for the Bay of Plenty report, NIWA scientists applied mid-range and high-range emissions scenarios.
The mid-range scenario describes a future world of very rapid economic growth. It assumes that the global population will peak mid-century, then decline, and that we will rapidly adopt new, more efficient technologies. It also assumes that our future energy will come more or less equally from fossil and non-fossil sources. In this scenario, carbon dioxide concentration doubles from pre-industrial levels by the 2060s, and is about 140 percent higher than pre-industrial (17th century) levels by 2100.
The high-range emissions scenario describes a more piecemeal response to climate change. Regions and nations adopt their own strategies and technologies, and their economies develop at differing rates. This slows the uptake of new technologies, while the global population goes on increasing. In this scenario, carbon dioxide concentration doubles from pre-industrial (17th century) levels by the 2060s and is almost 200 percent higher than pre-industrial by 2100.
Because scientists use a number of different climate models which can deliver differing results, they issue their projections within possible ranges. For example, when NIWA scientists say the Bay of Plenty is projected to warm by about 1.2 °C by 2040, 1.2ºC is the annual mean temperature within a range of between 0.5 and 1.5 °C.
By 2090, they expect that the region might warm, under the mid-range emissions scenario, by an annual mean of about 2.7 °C, within a possible range of 1.7 to 3.2 °C.
Under the high-range emissions scenario, they expect that the Bay of Plenty might warm by an annual mean of about 3.2 °C, within a possible range of 2.5 to 3.6 °C by 2090.
Apart from human influences on the climate, scientists also have to consider other, entirely natural, climate phenomena – such as La Niña and El Niño – that affect our climate.
In El Niño years, New Zealand tends to get stronger or more frequent winds from the west in summer, drying out east coast areas, but bringing more rain to the west. The 1997/98 El Niño drove one of the country’s worst droughts, that cost over a billion dollars in lost productivity.
La Niñas have a weaker impact on our climate, but tend to bring more frequent wet and warm northerly conditions to the Bay of Plenty. A La Niña brought record rain to the region over the first half of 2011.
It can be very difficult to tease apart the effects of climate change from Nature’s own variability. All these unknown quantities – human and natural – mean scientists cannot predict one definitive future scenario for the Bay of Plenty; they can only present a range of possible changes.
However, all those possibilities point to the same general warming trend, and researchers can say with some confidence which climate change scenarios are more likely, and which ones are less so.
Griffiths, G.;Mullan, B.; Ackerley, D.; Sood, A.; Carey-Smith, T.; Wilcocks, L.; Sturman, J. (2011) An updated climate change assessment for the Bay of Plenty – NIWA Client Report AKL-2011-019
Hannah, J.; Bell, R.G. (2012). Regional sea level trends in New Zealand. Journal of Geophysical Research–Oceans 117, C01004: doi:10.1029/2011JC007591
Hansen, J.; Ruedy, R.; Sato, M.; Lo, K. (2012) Global Temperature in 2011, Trends, and Prospects – GISS Surface Temperature Analysis. National Aeronautics and Space Administration
Bernstein, Bosch, Canziani et al. (2007) Climate Change 2007: Synthesis Report – Summary for Policymakers. Intergovernmental Panel on Climate Change