Climate Change Introduction

Philippines. Photo © TNC

What is Climate Change?

Climate change refers to the long-term changes in the climate that occur over decades, centuries or longer. It is caused by rapidly increasing greenhouse gases in the Earth’s atmosphere due primarily to burning fossil fuels (e.g., coal, oil, and natural gas).

These heat-trapping gases are warming the Earth and the Oceans resulting in rising sea levels, changes in storm patterns, altered ocean currents, changes in rainfall, melting snow and ice, more extreme heat events, fires, and drought. These impacts are projected to continue and in some cases, intensify, affecting human health, infrastructure, forests, agriculture, freshwater supplies, coastlines, and marine systems.


  • Atmospheric temperatures: 2-4°C increase by 2100, mostly due to human activity ref
  • Sea level rise: ~1 m rise by 2100 due to thermal expansion and glacial melting. Note: the contribution of the Greenland and West Antarctic ice sheet could increase the extent of sea-level rise ref
  • Changes in storm patterns - warming may cause tropical storms globally to be more intense on average (with intensity increases of 2-11% by 2100) ref

For more projections of climate change and specific impacts to coral reefs, click here. For local and regional, projections in key geographies, see the Resources section below.

Difference Between Weather and Climate

  • Weather refers to atmospheric conditions such as temperature and rainfall over a short period of time (a few hours or a few days). Weather is what you experience day to day.
  • Climate is the average pattern of weather for a particular place over a long period of time, usually at least 30 years.

Climate Variability

The natural variation in climate that occurs from month to month, season to season, year to year and decade to decade is referred to as climate variability (e.g., yearly cycle of wet and dry seasons in the western tropical Pacific).

Climate variability between years is caused by natural variations in the atmosphere and ocean, such as the El Niño Southern Oscillation (ENSO). ENSO has two extreme phases: El Niño and La Niña. El Niño tends to bring weaker trade winds and warmer ocean conditions near the equator across much of the Pacific, whereas La Niña tends to bring stronger trade winds and cooler ocean conditions.

Natural climate variability occurs in parallel with climate change (i.e., droughts and floods caused by ENSO will continue to occur and may intensify due to climate change). Therefore, these natural fluctuations must also be taken into account when planning for the future.

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