The climate and why it is changing

“Climate is what we expect, weather is what we get.” (Mark Twain)

Have you ever wondered how science can tell us what the climate will be in 2050? For many people forecasts of the weather for the next week are not very reliable. So how are we supposed to believe what scientists tell us will happen with the climate 30, 50 even 70 years ahead? Well, the answer lies in the difference between the weather and the climate.

Look outside your window and you could probably describe the weather. In scientific language ‘the daily local weather is determined by large-scale factors such as global atmospheric circulation, and small, chaotic factors such as storm activity at a particular time and location’ (IPCC, 2007, FAQ 1.2). Weather is something we directly experience as variations in the atmosphere. By contrast, the climate is a statistical representation of the average and variability of weather over a set time period, typically 30 years. As such, it is not something we can typically experience or comment on. Again, in scientific language ‘climate represents average weather. It is usually described in terms of the mean and variability of temperature, precipitation and wind over a period ranging from months to millions of years, but typically the period is 30 years’ (IPCC, 2007, FAQ 1.1).

This difference explains how we can say something about climate change while having limited confidence in weather forecasts a week, or two weeks, ahead of time. Whilst it is hard to believe anyone stating exactly what the weather will be outside my window in exactly two weeks time(to the minute, hour or day), there is an ability to have confidence in predicted trends such as the temperature being warmer or a predicted cyclone on its way across the country.

The atmosphere is a layer of gases that surrounds the earth. The atmosphere is highly sensitive to the initial temperature conditions of earth’s air, sea and land. Small differences in these conditions lead to changes in the weather. Because these changes are so sensitive and somewhat chaotic in the different types of weather produced, it is only possible to attempt to predict the weather two weeks in advance.

However, the climate is driven by large-scale factors such as the level of radiation received from the sun, the atmospheric composition and movement of currents in the ocean, such as the El Niño Southern Oscillation. These factors vary much more slowly than the atmosphere and so we can predict how they will change and by extension the general atmospheric conditions we can expect (such as warming temperatures).

Above: human enhanced greenhouse effect

The graphic illustrates how less heat escapes due to the human enhanced greenhouse effect, compared to the natural greenhouse effect.

Future changes in the climate

The Earth’s greenhouse gases trap heat in the atmosphere and warm the planet. The main gases responsible for the greenhouse effect include carbon dioxide, methane, nitrous oxide, and water vapour (which all occur naturally), and fluorinated gases (which are man-made gases that can stay in the atmosphere for centuries). Human activities produce these gases through such processes as burning fossil fuels (coal, oil and natural gas), manufacturing and agriculture.

Climate change is a reality. Human activity is changing the amount of greenhouse gases and therefore changing the natural and expected climate. Communities are already seeing hotter temperatures leading to drought, more rain leading to flooding and extreme weather events – “once in a generation” storms, cold waves and heat waves are becoming far more regular.

Currently human activity has produced greenhouse gases trapped in the atmosphere that has led to 1 degree of warming since pre-industrial time (1820). Highlighting the sensitivity of the atmosphere, two degrees of warming was decided as the boundary between dangerous (crises, disasters, significant risk) and non-dangerous (manageable, less risk) climate change.

Significant strides have been made by the governments around the world in agreeing to limit how much greenhouse gases will be produced and emitted into the atmosphere in the future. However, according to projections of future climate, there is still only a 50 percent chance of keeping global warming to below 2 degrees (Meinshausen et al. 2022). However, this threshold has changed recently to 1.5 degrees as it is clear that the risk of catastrophic impacts of climate change are already emerging.

In questioning just how bad the future climate might be, we still need to remind ourselves that there is much uncertainty in projecting the future climate. For example, there is uncertainty due to the chaotic nature of the atmosphere. There is uncertainty according to natural events such as volcanic eruptions. In understanding future climates, there is also uncertainty from the different climate models used. Probably, the biggest uncertainty is due to the future changes in how much greenhouse gases we produce. Putting these, and other uncertainties, together means we have a range of possible climate futures we need to consider. Another important uncertainty is how global climate changes translate to local changes.

However, given these uncertainties there are some things we can have confidence in about the future climate. Given ongoing human activity:

• The concentrations of greenhouse gases will keep increasing
• These will cause global temperatures to keep going up (getting warmer)
• As snow and ice over land melts and the sea warms, sea levels will keep rising
• The carbon dioxide in the atmosphere will increase
• The oceans will become more acidic affecting sea life
• The climate will get more extreme, leading to more extreme weather events such as hotter days, intense rainfall (flooding or extreme storms), interspersed by periods of no rain leading to more drought

Reference

Meinshausen, M., Lewis, J., McGlade, C., Gütschow, J., Nicholls, Z., Burdon, R., Cozzi, L. and Hackmann, B., 2022. Realization of Paris Agreement pledges may limit warming just below 2° C. Nature, 604(7905), pp.304-309.

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Localising climate projections