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All about Climate

Objective: Look closely at the different definitions of 'climate', and why it is so complex to describe. Go through the key concepts of 'mean' and 'variability' in the climate system and what they mean for the world today.

The word 'climate' is now widely used not only in science, but also in politics, industry, and many aspects of everyday life. Despite this common usage, the definition of 'climate' is less easy to tie down or explain in a few words. For example the Intergovernmental Panel on Climate Change (IPCC), in many ways the international authority on climate change, defines climate in the following manner:

"Climate in a narrow sense is usually defined as the average weather, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization. The relevant quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system. In various chapters in this report different averaging periods, such as a period of 20 years, are also used."

Essentially, 'climate' is defined as 'average weather'. The question then becomes what period of time we average over: days, months, years? A time period of 30 years is usually used here, but in some cases this is not appropriate. Say we wish to examine whether a particularly extreme storm is usual for the given climate, or if it suggests that climate itself has changed. Averaging over 30 years of data may remove this brief but extreme event from the data, and incorrectly suggest that no changes have occurred.

Another example where we may want to average over longer timescales is when considering the change from ice-age conditions to ice-free conditions over geological timescales. Averaging over 30 years would be far to short in this case and we may wish to look over a hundred or in some cases even thousands of years. These examples are not meant to be confusing, simply to show that climate is an idea that can't be easily summed up in one sentence.

Let's take a closer look at the IPCC definition, which states that climate may also be defined more rigorously as "the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years". This is not so easily digestible as "average weather", but includes some important concepts.

To look into this in more detail let's focus on one aspect of climate: temperature. Other key variables may be rainfall, sea level, the volume of ice, wind speed, etc... Weather is the state of the atmosphere or earth's surface at a given time and place. Unlike climate which we may expect to remain constant with no change in external forcings, weather is constantly changing. One easy example of this is a day: it usually is warmer during the day than during the night.

Change in temperature over the course of a day ('weather'). The purple line shows the average temperature over the course of the day. Despite being 'average weather' this cannot be called climate.

So why is 'average daily temperature' not climate? I am sure you all have an intuitive idea of why this is, we are considering a period of time that is simply not long enough. We can visualise this by looking at the annual temperature change. Winters are cooler than summers, and no one would consider warming from winter to spring to be 'climate change'. Our daily average cannot be counted as it does not account for this annual variation.

Change in temperature over the course of a year ('average weather'). The purple line shows the average temperature over the course of this year. What do you think here, can this annual average be considered 'climate'.

Taking an annual average has removed the daily and yearly trends from our data. In some cases, yearly averages are considered 'climate', but this is often risky due to the natural variability of our atmosphere. Again with no change in external conditions, the yearly temperature will fluctuate around a mean value. Comparing yearly averages alone can miss this and lead to interpretations of trends that are simply natural variability.

Change in annual mean temperature over the course of 50 years. The purple line shows the trend over the entire 50 years, which in this case can be considered a change in climate. See the case in the box of where a trend from too few years may be misleading due to internal variability.

Averaging trends over decades is an effective way of removing both annual trends and removing the internal variability of data. However we are missing information here, as it is not only the mean temperature which is important. Extreme high and low temperatures are almost always the most dangerous, and their timing is also important. 10 very hot days spread throughout a summer may not do much damage, but if they are all grouped within two extreme weeks of heatwave, more damage may be inflicted. To take this into account but still average out daily and annual trends and the data variability, we must consider some statistics of the weather dataset. These include and are not limited to minimum and maximum, standard deviations, median, etc. By considering changes in not only the mean but also these quantities over time, we may begin to capture more details of the climate system.

Example of two years from the 50-year trend with identical mean annual temperatures but different overall trends. The later year shows a less even variation with a sharp summer peak in temperatures. The red lines show the 90th percentile high temperature for the year, see how this may pick out the trend where the mean annual temperature could not.

To sum it up, climate represents the average of weather (i.e. the temperature, rainfall, wind etc conditions) over a representative timescale that is usually several years, but may also in some cases be longer. As such, climate change is a change in these average conditions. This change may be an increase in mean temperature, but may also be a change in variability, an increase in absolute values, etc.

All four plots show a change in annual temperatures between a prior condition (blue) and a later condition with a changed climate. In 1 there is a simple increase in mean temperature. In 2 the mean temperature remains the same, but the variability increases. In 3 both the mean and variability increase. In 4 the mean and variability are unchanged, but the seasonality is shifted (high temperatures occur earlier in the year).

There are many ways in which climate can change, and a change in one parameter (eg: temperature) may or may not also be associated with changes in another (eg: rainfall, sea levels, etc.). Given the complex world we live in, each change is associated with many effects for both the natural system and the humans that live around it. Climate change is often simplified down to a few simple numbers (2 degrees climate warming, 50 cm sea level rise) which make for easy to communicate and debate targets. They however hide the complexity of the problem: a 50 cm rise in sea levels associated with a change in variability may also mean that annual storms surge levels are 3 m higher. 2 degrees of global warming may actually only warm Europe by 1.5 degrees on average, but warm parts of the Arctic by more than 5 degrees.

Many other factors influence these changes, with humans being one of the most critical in today's world. The objective of this website is to examine how two other factors, glaciers and volcanoes, interact with the climate. While not the primary drivers of current warming, increase in carbon dioxide or sea level rise, they have the potential to modulate much of the rate and magnitude of human-induced changes in the climate system.