What is climate change?

Climate change is defined as a long-term change to the Earth’s climate. But what does this mean? To understand climate change, we need to understand what makes up our climate.

Climate versus weather

There is a saying that climate is what you expect, weather is what you get.

Weather is what is happening right now. It is the current atmospheric conditions – temperature, rainfall and wind happening in a specific location or region. The weather can change daily or even hourly.

Climate is a combination of weather conditions that occur throughout the year and is averaged over decades – 30 years or longer.

There are many factors that influence climate – nearness to the ocean, elevation, prevailing wind patterns and topography – so climate patterns of rain and sunshine differ from place to place. Topography is one of the key factors in Aotearoa – the mountain ranges that run the length of the country divide the land into climate regions. Areas like the Bay of Plenty, Marlborough and Nelson are sheltered from prevailing westerly winds and tend to have drier more-settled climates than those on the west coasts. Western regions often have stronger winds and higher rainfall, especially along the coastline.

Climate can be variable

We know the weather can change daily and even several times during the day. Our climate can also vary in response to natural influences. Aotearoa experiences climate oscillations that influence both our weather and climate. A climate oscillation is a natural recurring pattern of changes in air pressure, sea temperature and wind direction. Aotearoa is subject to a few of these. One example is the El Niño Southern Oscillation. This is a change in the movement of wind and warm water across the Pacific Ocean, and we feel its effects every few years with changes in rainfall patterns.

Other natural climate variabilities include ‘wobbles’ in the Earth’s orbit that create ice ages every 100,000 years, sunspots that occur every 10 years or so and large volcanic eruptions that can cool the planet for a couple of years.

Human influence

Our climate is also changing due to human influences. The greenhouse gases in our atmosphere hold in heat, protect us from harmful radiation and sustain plant and animal life. Without these gases, Earth would be a barren, rocky planet.

A balance of atmospheric gases has kept the annual average temperature in Aotearoa within a range of 10–14°C for the last 14,000 years – the time of the last ice age. For the past 3,000 years the average temperature in Aotearoa has been relatively steady, rising and falling by less than a degree over this time.

Our burning of fossil fuels and other activities have increased the amount of greenhouse gases in the atmosphere. Greenhouse gas emissions started to increase 200–300 years ago when we began to burn fossil fuels at the start of the industrial revolution. In recent decades, emissions have risen even more rapidly. It takes a lot of energy to power our modern lifestyles!

We are already seeing changes in patterns of rainfall and temperature across Aotearoa, with increasing extreme rainfall as well as frequency and intensity of droughts in some places. The annual average land-surface temperature in Aotearoa has risen by 1.1°C since records began in 1909. That doesn’t sound like much of an increase, but small changes can have large consequences.

It has been 10,000 years since the average annual temperature in New Zealand was this warm, which is likely to be near the edge of the range that current ecosystems have experienced here.

Our atmosphere and climate 2020

Evidence of human influence

How do we know that humans are responsible for the marked increase in greenhouse gases? One line of evidence comes from scientists who are able to ‘fingerprint’ and separate natural emissions from those caused by humans. The carbon in two of the greenhouse gases that come from fossil fuels – carbon dioxide and methane – is chemically different from atmospheric carbon that enters from the natural carbon cycle.

Scientists also use other methods to gather data as evidence that changes are due to human actions. Learn about these methods in the articles Carbon dioxide and climate and Clues to the past.