Understanding Climate Change Science

This page provides an overview of climate variability, extremes and change. For more detailed information, the reader is referred to the resources available at www.pacificclimatechangescience.org.

Climate variability and change

Each region of the world has its own unique climate, which is the typical weather the region experiences. However, the weather varies from month to month, season to season and year to year due to natural cycles and the influence of large-scale climate features like the El Niño–Southern Oscillation (ENSO).

ENSO is the major cause of year-to-year climate variability in the Pacific. The extent and timing of its influence vary between countries. Throughout the region, ENSO affects the year-to-year risk of droughts, floods, tropical cyclones, extreme sea levels and coral bleaching.

Climate change occurs over much longer timescales than climate variability—decades, centuries or longer. It is the result of both natural processes and human activities. It can mean long-term changes in the average climate conditions (such as average rainfall and temperature) or in the occurrence of extreme events such as tropical cyclones and droughts.

Human activities change the climate by increasing greenhouse gas levels in the Earth’s atmosphere. Greenhouse gases occur naturally in the atmosphere and trap heat. However, human activities like burning fossil fuels (such as coal, oil and natural gas) are rapidly increasing the concentration of these gases in the atmosphere. This is causing the climate to become increasingly warmer and weather patterns in some places to change.

Climate extremes

Climate extremes are short-term weather or longer-term climatic events that are rare or uncommon in occurrence, and often excessively severe in impact. Extreme events resulting from natural variability in large-scale climate processes, from season to season and year to year, can cause massive loss and damage to infrastructure, industry and environmental assets, and can impact on the health, safety and overall wellbeing of local communities. These large-scale processes include ENSO, the South Pacific Convergence Zone, the West Pacific Monsoon and the Intertropical Convergence Zone. Longer-term variability and climate change compound these impacts, particularly in terms of increased vulnerability to natural, climate-related disasters.

The impacts of climate change will be felt through extreme events (changes in frequency and intensity) rather than changes in mean conditions. Consequently, climate risk assessments will, for the most part, focus on changes in extreme events.

Climate models
Global climate models

Global climate models are mathematical representations of the Earth’s climate system. They are run on powerful computers to simulate the processes affecting weather and climate. There are many global climate models being used, and there is no single ‘best’ model. The group of current, good-quality model simulations are brought together and compared in the Coupled Model Intercomparison Project (CMIP), and the latest version in CMIP5.

Model biases

The performance of the model in the past and present climate is a general guide as to how confident we are in the projections. The difference between a model and observations is called bias. If models have low biases in the important aspects of performance, then confidence is higher. If models have high biases, then confidence is lower. Models with high biases in important features may be rejected. For example, in the PACCSAP program, three of the 27 CMIP5 models that were assessed were deemed as unsuitable for making projections in the western Pacific and so were rejected.

We now have a good understanding of many aspects of the climate system but our understanding and ability to model the current climate is not perfect. Also, our understanding and modelling of how the earth responds to an enhanced greenhouse effect are not perfect.

Unfortunately, some of the biggest biases in climate models are in the western Pacific! This means that confidence in some projections is reduced compared to other regions. Particular biases include:

  • The West Pacific Warm Pool and equatorial ‘cold tongue’ can be the wrong shape – the cold tongue is too strong in models
  • The rainfall zones South Pacific Convergence Zone and Inter-Tropical Convergence zone can be too strong and lined up the wrong way

Some aspects of climate change are not currently clear, or we are not confident about, including:

  • Patterns of rainfall are different with a warmer climate, but depend on many different processes, so the exact pattern of regional rainfall change is not clear
  • Change in the variability and intensity of the El Niño Southern Oscillation is not completely clear
Climate change projections

We do not know how climate change will affect natural climate variability and extremes, or how greenhouse gas and aerosol emissions will change in the future, or how the climate will respond to changing emissions. For this reason, there will always be a range of uncertainty in climate projections. (It is worth noting that while non-scientists use ‘uncertainty’ to describe things that are unknown, climate scientists use the term to describe what they do know – so uncertainty in climate projections is not a ‘bad’ thing, it just tells us the limitations of the information we have.)


Natural ups and downs in the climate will always continue. There will also be unpredictable things that affect our climate, such as large volcanic eruptions. Human effects may be small or large compared to the natural changes:

  • Natural effects are more important the more local and the shorter time frame you look at – such as what will be the temperature of my island one year to the next.
  • Human effects become more noticeable the wider the area and longer the time frame you look at – such as the temperature trend in my country in the next 50 years.


We don’t know for sure how much carbon dioxide and other gases people will emit into the air – it depends on how society and technology develop, and whether international agreements are successful. We must make projections for various scenarios of emissions, from low to high, and we must use projections knowing that they are not an exact prediction of what will definitely happen. You may wish to show the effect of a high scenario as a worst case, or show a high and a low scenario to show the effect of controlling emissions.

Climate response

We know that increases in greenhouse gases like carbon dioxide make the world warmer, but the Earth’s climate is very complex and there are a lot of things we don’t have a clear idea of yet. A range of model results gives us an estimate of the range of possible change, but on top of this it is important to understand the confidence in projections, from low to very high.

Confidence ratings

Projections are given a confidence rating, generally from low where model biases are significant and the processes involved are less certain, through to very high confidence, where we have confidence in the models and understand the processes well.

There is presently greater confidence in projections of some variables (e.g. temperature) than others (e.g. rainfall), and greater confidence in projections over large spatial scales and long time periods (e.g. global climate change over multiple decades) than for smaller spatial scales (e.g. regional and national projections) and short time periods (e.g. over periods of less than ten years).

Projections of average temperature are more confidence than rainfall.

The confidence rating should be used to inform how the projections are used – if confidence is high, then the ranges of model results can be used as a good guide to potential climate change, if confidence is low then the results are plausible but other possibilities should also be considered possible.

IMPORTANT: Do not use results with too much or too little confidence – climate projections are guides to what the future may look like under future scenarios, they shouldn’t be used as precise numbers, but neither should a projection be treated as not useful.