Warming in Wellington

By Gareth Renowden 22/07/2012


In this guest post, first published last week in the Dominion Post, Jim Salinger looks at the long term temperature record for Wellington, and how it has been constructed. Jim’s currently the Lorry Lokey Visiting Professor in the Program in Human Biology at the Woods Institute for the Environment at Stanford University in California.

Climate scientists want to monitor how climate is changing and global warming progressing. This is particularly pertinent as this week the New Zealand Climate Science Education Trust are currently being heard in the Auckland High Court to try to persuade a judge to invalidate New Zealand’s temperature records which have been compiled and collected by the National Institute of Water and Atmospheric Research and the former government agencies. The coalition asserts the only way NIWA can claim a warming trend of 1°C over the past century is by cooking the books.

This century climate scientists are very interested in tracking climate as human factors are going to be the dominant influence on climate this century, save a meteor crashing in to the planet. They are interested in adjusting the readings as though they are taken from one location in an area. Wellington has one of the longest and best climate records of any region in New Zealand. This is why climate scientists carefully adjust temperature records.

When Sir James Hector, Director of the Colonial Museum in Wellington in the 1860s established a network to monitor New Zealand’s weather and climate, the primary stations were established for weather forecasting, so the priority on permanency of location of a climate monitoring site for climate change was lower. However we are indebted to Sir James’s Scottish heritage as in setting up the network he purchased precision thermometers which were housed in Stevenson screens to ensure consistency. Observations were taken under standard conditions, in his words ‘rigorous….’. This has given us a legacy of climate monitoring under rigorously enforced methods with very reliable observations from the 19th century, the envy of many countries.

Climate scientists, in constructing a climate record over the last 150 years for Wellington have to adjust the measurements taken for a number of reasons. The climate record has been taken from not one, but five sites. The various sites are at different locations in the area of Wellington City. These sites have different temperature characteristics and may be cooler or warmer than other sites because of factors such as different elevation above sea level, or be in city centres. By adjusting temperature series changes that are caused by climate, rather than the changes in site or environment can be monitored.

The diagram shows the record of mean temperature at the five sites: Knowles Observatory, halfway between the harbour and Tinakori hills; the Government Astronomical Observatory on a hill in the Bolton Street cemetery; Buckle Street; the Thorndon Esplanade; and then the current one at the top of the Botanical Gardens at Kelburn, 125 metres above sea level. This is the highest, and coolest of all the Wellington City long-term climate recording sites.

WellingtontempsFig1

Figure 1. Unadjusted climate observations in Wellington City 1863-2011 from five different monitoring sites. The mean annual temperature is in °C. Wellington Airport is also shown for comparison.

To obtain a consistent record to monitor global warming locally adjustments are made so that the readings reflect those at the current site. To make these changes climate data are rigorously checked for any obvious errors. Adjustments are made by comparing a period of overlap between the old and new site, and comparing climate data before and after the site change with other neighbouring climate stations.

By these means a number of adjustments to the mean temperature have been calculated for the Wellington sites. All earlier sites are warmer than the current well ventilated Kelburn site by as much as 1.0°C because they are much lower in altitude.

It is from Wellington’s adjusted long-term record that true climate trends can be obtained. The long term record from Wellington City thus calculated shows that there is year to year variability which can be as much as 1.5°C between years. However there is an overall clear trend with Wellington mean annual temperatures now 1.3°C warmer by 2011 than in the early 1860s. This has been noticeable in the ability of gardeners to now grow frost sensitive plants in warmer parts of Wellington. Both the shorter climate series from the single sites at Palmerston North and Westport show extremely similar trends and variations, with mean annual temperature increases of about 0.8°C from the 1930s to 2011.

WellingtontempsFig2

Figure 2. Long term climate series from Palmerston North, Westport and the combined record for Wellington City. Arrows indicate site changes at Wellington City. Annual mean temperature °C.

Long term monitoring is essential to detect small, but significant changes in climate. As records are taken from a number of different sites in a locality these require adjustments to reflect the true climate trends. Wellington’s temperature readings have been carefully adjusted with each site change to provide a consistent and reliable long-term record. The excellent climate record from Wellington shows a clear warming trend over the past 150 years.