Global mean surface temperatures have increased by 0.4-0.8 ^oC since the second half of the 19^th century, with the ten globally averaged warmest years all occurring since 1983 (Karl et al., 1999). At the same time there have been clear precipitation trends for parts of the globe. Observed trends and variability in climate and sea level for Oceania (Australia, New Zealand and the South Pacific) show that annual surface air temperatures increased between 0.4-0.8^oC throughout most of the region in the period 1951-1993 (Salinger, et al., 1996), with precipitation changes.

This change has provided a background on which climate shifts, driven by the recently discovered process the Interdecadal Pacific Oscillation (IPO) has occurred. The IPO operates on time scales of two to three decades, and causes major shifts in Oceania climate (Power et al. 1999, Salinger et al. 2000). This paper will identify the main trends and variability in climate that have occurred in global and Oceania climate during the 20th century, the changes, those that are important for primary producers, regional and district councils, resource managers and those involved in land-based industries. It will conclude that climate is changing.

1. Introduction

Climate change and variability have occurred throughout time. All evidence over the last glacial/interglacial period indicates that global temperatures have varied in the order of 5°C over the last 20,000 years (ref), and that the climates of Australia, New Zealand and the South Pacific show trends similar to those of the globe.

This paper will examine the evidence of how our climate has been changing during the 20^th century, the longer-term trends that have been occurring, and the shorter-term fluctuations that operate over several decades.

Accurate global measurements of surface temperatures commenced during the 19^th century. These observations are from long term sites of land surface air temperatures where the records have been carefully adjusted, combined with sea surface temperatures measured by ships, and more recently from satellites. These records clearly show that land surface temperatures have increased by 0.4 – 0.8°C since the late 19^th century (Figure 1). Two period of warming occur: that over the period 1910-1945, and the other from 1976 onwards.

In the Southern Hemisphere, there was less warming during the 1910-1945 period, with that in the later period being more sustained.

Of particular note have been global temperature trends in this latter period. The ten warmest years in the instrumental record have all occurred since 1983, with 1998 being the warmest year on record (Karl et al., 1999). The recent increase in global temperatures since 1976 has been at a rate of 0.2°C a decade. The temperature trends have been accompanied by changes in land surface precipitation. There has been an increase in precipitation in many mid and high latitude areas of the Northern Hemisphere, whilst many areas in the subtropics have shown a decrease.

In Australia, a positive trend of typically 1°C has occurred in minimum temperatures at nearly all 223 station records examined over the period 1910-1993 (Salinger et al., 1996). The trends in maximum temperature are more spatially variable, with negative trends through eastern New South Wales. From a total of 341 high quality records, the time series of All-Australia averages of annual rainfall show that it has been greater since about 1950 than earlier in the century. This is due to mainly increased summer rainfall in northeastern Australia, due to an increase in precipitation events.

In the South Pacific, including New Zealand mean island near-surface air temperatures have increased by between 0.3 to 0.8 ^oC during the 20^th century, with the largest increase in the zones south west of the South Pacific Convergence Zone (SPCZ). Marine surface temperatures show parallel trends. In regions north east of the SPCZ most the warming occurs since the mid-1970s, whilst regions to the southwest show steady warming throughout the second half of the 20^th century. The SPCZ represents a pivotal line for long-term changes in annual rainfall, with increases in annual totals in those areas to the north east, and decreases to the south west after the mid 1970s.

3. Interdecadal Pacific Oscillation

Recently shifts in climate have been detected in the Pacific basin, driven by a newly described climate feature, the Interdecadal Pacific Oscillation (IPO), which shifts climate every one to three decades (Power et al, 1999). This is an ‘ENSO-like’ feature of the climate system that operates on time scales of several decades. There is a tight coupling between the ocean and atmosphere. The main centre of action in sea surface temperatures is in the north Pacific centred near the dateline at 40 ^oN, with an opposing weaker centre just south of the equator in the eastern Pacific north the Easter Island at 10 ^oS. There is also another weaker centre of action, in the south west Pacific centred near the Cook Islands at 20 ^oS, which is in the same phase as the north Pacific centre. The matching atmospheric sea level pressure (SLP) pattern is one of an east/west seesaw at all latitudes, but again centred over the north Pacific, with the centre of action over the Aleutian Islands. During the negative phase of the IPO the opposite sea surface temperature anomaly and SLP patterns to those described occur.

Three phases of the IPO have been identified during the 20^th century (Figure 2): a positive phase (1922-1944), a negative phase (1946-1977) and another positive phase (1978-1998). When the IPO changes phase, shifts in average climate occur in parts of Oceania (Salinger et al., 2000). The evidence of the last 24 months suggests that the region may now be entering another of these IPO swings in climate, which will require a few more years to confirm.

Figure 2. Index denoting the phases of the IPO.

In Oceania SLP in the west of the region increased for the most recent positive IPO period, and decreased east of 170°W, with generally more southerly quarter airflow than in the previous negative phase. Annual surface temperature increased over Australia and the south west of the oceanic regions at a rate similar to the average Southern Hemisphere warming. However, in the northeast of the region temperature increases were more. Increases in annual precipitation by 50% or more occurred in this northeastern region, compared with smaller decreases elsewhere.

The IPO strongly modulates year-to-year ENSO variability. For the positive phase, interannual relationships between ENSO and Australian precipitation and temperature weaken (Power et al., 1999). The IPO modulates these teleconnections in a complex way throughout the South Pacific, strengthening relationships in some areas and weakening them in others. Only for New Zealand is there a consistent bias towards stronger relationships for the positive IPO period, the reverse of Australia.

The results presented in this paper have described longer-term change as a result of global warming, and shorter-term climate shifts associated with the most recent transition from the negative to positive phase of the IPO. The longer-term trends have been occurring over the entire 20^th century in the Oceania region. These trends provide the background upon which decadal scale changes occur because of the IPO. The change from the most recent negative to positive phase of the IPO has produced mean changes in climatic elements that are substantial. The IPO also modulates ENSO relationships throughout Oceania, with Southern Oscillation Index-climate correlations strengthening in some parts of the region, and weakening in other parts.

Managing variability and change requires climate-proofing activities to increasing variability, shifts and changes from decades and long-term change. The evidence from the 20^th century shows that long-term change because of global warming has been occurring in the region at the rate of 0.2°C per decade. On this background the IPO causes major shifts in climate over periods of decades. Adaptation of land-based activities to increasing climate variability will provide some protection to long-term change. However, the IPO shifts provide a challenge to land-based industries with abrupt changes in climate means. The 20^th century evidence is that the climate system is changing.

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Power, S.B., T. Casey, C Folland, A Colman, and V Mehta, 1999: Inter-decadal modulation of the impact of ENSO on Australia. Climate Dynamics, 15, 319-324.

Salinger, M.J., Allan. R., Bindoff, N., Hannah, J., Lavery, B., Lindesay, J. Nicholls, N., Plummer, N and S. Torok. 1996. Observed variability and change in climate and sea level in Australia, New Zealand and the South Pacific. In Greenhouse: Coping with Climate Change (eds) Bouma, W.J., Pearman, G.I and M. R. Manning. CSIRO, Melbourne, 100-126.

Salinger, M.J., Renwick, J. A. and A.B. Mullan, 2000: Interdecadal Pacific Oscillation and South Pacific climate. International Journal of Climatology (submitted)