Natural hazards

Climate change

Climate is changing. Future changes in temperature, precipitation, and other climate variables will alter Auckland's soil moisture and mean sea level. Some locations will be more susceptible to floods and droughts.

Global trends

"An increasing body of observations gives a collective picture of a warming world and other changes in the climate system" - from the Third Assessment Report of the Intergovernmental Panel on Climate Change (2001). This report summarises other global climate trends:

Temperature

Global average surface temperatures have risen since 1861 with an increase over the 20th century of 0.6 ± 0.2°C.

Globally, the 1990s was the warmest decade and 1998 the warmest year in the instrumental record since 1861.

The temperature increase in the Northern Hemisphere during the 20th Century is likely to have been the largest in any century during the past 1,000 years.

1950 - 1993, night-time daily minimum air temperatures over land increased by about 0.2°C per decade. This has lengthened the freeze-free season in many mid- and high latitude regions has been lengthened.

There has been a reduction in the frequency of extreme low temperatures since 1950, with a smaller increase in the frequency of extreme high temperatures.

Since the late 1950s global surface temperatures have been increasing at 0.1°C per decade.

Extent of snow cover and glaciers

Since the late 1960s, snow cover has decreased ~ 10%.

Mountain glaciers retreated in non-polar regions during the 20th century.

Sea level changes

Global average sea level has increased 0.1-0.2m during the 20th century.

Precipitation

Unlike the Northern Hemisphere, no comparable systematic changes in precipitation have been detected over the Southern Hemisphere.

Auckland climate trends

Temperature

New Zealand has three regions of similar temperature patterns (Salinger and Mullan (1999)). The temperature in the Auckland region varies in similar ways to much of the central and northern regions of the North Island:

1930-1945
lower temperatures observed in response to a more southerly airflow.
Mid- to late-1940s
increasing temperature trend.
1951-1998
increasing temperature trend, although increases were greater during 1951-1977, at a time of positive Interdecadal Pacific Oscillation (see below).

Decrease in the number of cold nights.

The warmest temperatures on record were experienced in 1988-1990 (Daw and Salinger, 2000; Salinger and Daw, 2001).

Precipitation

The temperature in the Auckland region varies in similar ways to much of the northern North Island. Rainfall trends for this northern North Island region have been through three stages:

1930-1950
Near average rainfall (enhanced El Nio conditions & increased westerly circulation (positive IPO))
1951-1975
high rainfall (enhanced La Nia and north-easterly flow (negative IPO))
1976-1994
low rainfall (enhanced El Nio conditions & increased westerly circulation (positive IPO))

Sea level

1899-1988 - Average sea level rise of 1.3 mm/yr. (Hannah, 1990).

However, within this record are periods of near static and apparent acceleration in mean sea level rise that correspond with different phases of the IPO. Mid-1970s - near static trend in sea level, whereas for the 40 years prior to the mid-1930s - mid 1970s (time of shift) sea level "appeared to accelerate".

Interdecadal Pacific Oscillation

The Interdecadal Pacific Oscillation (IPO) seems to accentuate or curtail the effects on El Nio and La Nia events depending on its phase (Bell, et. al., 2000; Salinger, et. al., 2001).

The IPO oscillates between a 'positive' and 'negative' phase every 20-30 years which coincides with changes in the New Zealand's climate regime (Salinger and Mullan, 1999; Salinger, et. al., 2001).

Positive IPO is associated with enhanced El Nio conditions, and Negative IPO is associated with normal fluctuation between El Nio and La Nia conditions.