Changes in surface climate of the tropical Pacific

Based on…….

Outline and key messages Main features of region’s climate system We are affecting the climate system - climate is already changing Future will be warmer Extreme weather likely to be more extreme Our climate will be changing for foreseeable future

Main climate features of the region

Pacific atmospheric circulation Trade winds Convergence zones Walker and Hadley circulations

Sea surface temperature

El Niño-Southern Oscillation (ENSO) Major source of year-to-year climate variations

Typical El Niño and La Niña rainfall patterns Wetter (green) or drier (orange)

Typical El Niño and La Niña rainfall Annual rainfall – Pehnryn

Typical El Niño and La Niña temperature Annual temperature – Rarotonga

Effects of El Niño and La Niña on SST Warmer (red) or cooler (blue)

ENSO shifts SPCZ and tropical cyclones El Niño Further north El Niño Further south La Niña La Niña Fewer cyclones further east El Niño More cyclones further west La Niña

Cyclones in Cook Islands Number of cyclones passing within 400 km of Rarotonga

Projected changes in climate

Redistributing sun’s energy = climate system ADD digram Without the atmosphere the Earth would be ~30oC cooler More greenhouse gases trap more energy in climate system

Why are climate scientists so sure climate is changing due to human activities? Theory Modelling Evidence: instrumental measurements changes in the physical world changes in the biological world paleoclimate archives The climate system appears to be changing faster than earlier thought likely Steffen 2009

Measured increase in carbon dioxide 18th century = 280ppm air bubbles in ice cores 2011= 391ppm Mauna Loa

Observed warming of global temperatures

Projecting future climates Emissions Scenario IPCC-AR4 (2007) Temperature (oC) Rainfall 2035 2050 2100 Low (B1) 450-500 ppm CO2 0.5-1.0 ? 1.0-1.5 5-15% 10-20% High (A2) 750-800 ppm CO2 2.5-3.0 5-20% Predicting future forcing – how much more greenhouse gases? Range of possible futures but the future will be WARMER

Spatial variation in warming

Cook Island’s temperature projections Northern Group - increase in temperature of 0.5 – 0.9oC Southern Group - increase in temperature of 0.4 –1.0oC More hot days and warm nights Decline in cooler weather.

Spatial variation in rainfall

Cook Island’s rainfall projections Uncertainty around rainfall because model results are not consistent but generally projected to increase For the Southern Group, average rainfall during the wet season is expected to increase due to intensification of SPCZ Droughts are projected to become less frequent throughout this century Reasons for moderate (bordering on high) confidence in wet season rainfall increase: Positive: An increase in wet season rainfall is consistent with the projected likely increase in the intensity of the South Pacific Convergence Zone (SPCZ), which lies over Vanuatu in this season (the majority of CMIP3 models show a stronger SPCZ) Positive: Models get SPCZ in the correct location relative to Vanuatu Negative: Models aren’t as good at rainfall as they are at temperature Reason for moderate confidence in dry season rainfall increase: Positive: Approximately half of the CMIP3 models agree on this direction of change by 2090 (depending on exactly which emission scenario you look at) Negative: In simulations of the current climate, some CMIP3 models have an SPCZ that extends too far east during the dry season, with too much rainfall over Vanuatu (Brown et al., 2011). Reason for low confidence in annual rainfall increase: Projections of annual mean rainfall tend to be equally divided between an increase (>5%) and little change (-5% to 5%) by 2090, with only a few models simulating a decrease (<-5%). There is only moderate and low confidence in the range and distribution of wet and dry season rainfall projections respectively, as discussed above. Reasons for high confidence in extreme rainfall increase: An increase in the frequency and intensity of extreme rainfall is consistent with larger-scale projections, based on the physical argument that the atmosphere is able to hold more water vapour in a warmer climate (Allen and Ingram, 2002; IPCC, 2007). It is also consistent with the projected likely increase in the intensity of the SPCZ (Volume 1, Section 6.4.5). Reasons for low confidence in drought projections: There is only low confidence in the range of dry season rainfall projections (Section 16.7.2), which directly influences projections of future drought conditions.

02/03/11 02/03/11 Cyclones Cook Islands projections indicate there is likely to be a decrease in the number of tropical cyclones by the end of the 21st century But there is likely to be an increase in average maximum wind speed of cyclones by between 2% and 11% A list of the projections being delivered in the PCCSP. 24

Extremes The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be Trenberth 2012

Important points to note Extremes will become more extreme Unclear how ENSO will change – continued influence

For foreseeable future, climate will be CHANGING Summary Not just a ‘new climate’ to which we can adapt……. For foreseeable future, climate will be CHANGING