Evidence for Change

Nobody was there with a thermometer

The climate has swung between long cold periods and warmer ones for as far back as anyone can reconstruct. The interesting question at GCSE is not that it changed. It is how we know. Direct measurements only go back a couple of centuries. Everything before that comes from proxy records: things that were not made to record the climate but did so anyway, because the climate changed them. This module covers the past changes, the natural causes behind them, and the four kinds of evidence. It also does one thing the examiner asks for explicitly: keeps natural causes and human causes in separate boxes.

Six words to be exact about

Precision here is worth marks, because two of these are routinely used the wrong way round:

Which are we in now?

The Quaternary has alternated between glacials and interglacials. Where does the present sit?

• An interglacial: a warmer period between colder glacial ones
• A glacial, since ice sheets still exist in Greenland and Antarctica
• Neither: the cycles stopped at the end of the last ice age
• Both at once, since some regions are cooling while others warm

Two boxes, and do not mix them

The examiner asks for natural and human causes to be kept in clearly separate categories, and mixing them is the fastest way to lose a straightforward mark. This module is about the left-hand column. The right-hand one is the subject of the specification's own section on present-day change, and all you need here is to recognise which box a cause belongs in.

Match each natural cause to how it works

Which TWO are natural causes?

Sort these into the right box. Select the TWO that are natural causes of climate change.

• A slow change in the angle at which the Earth leans on its axis
• A large eruption putting ash and gas into the upper atmosphere
• Methane released by intensive livestock farming
• Carbon dioxide from road transport and aviation

Four kinds of evidence, and the trade-off

Ice cores. Ice builds up in annual layers and traps bubbles of the air that was there at the time. The trapped gas and the chemistry of the ice itself both carry a temperature signal. Reach: very long. Precision: good near the top, coarser with depth as the layers compress. Tree rings. One ring per year of growth, and the width of each depends on the conditions that year. Reach: a few thousand years at most. Precision: year by year, the best there is. Pollen records. Pollen preserves in peat bogs and lake sediments, and different plants live in different climates, so the mix of species shifts as the climate does. Reach: long. Precision: broad. Historical sources. Paintings, diaries, harvest and shipping records. Reach: a few centuries. Precision: detailed, but written by people with opinions. So there is no best evidence. Every proxy trades reach against resolution, and the question you are answering decides which one you want. Next you will read a tree-ring record yourself.

Plot the ring widths

Here are ring widths from an example core sample, invented for this module. Width in millimetres is up the graph, and the year of growth runs across, 1 to 6. The widths are: year 1 is 4, year 2 is 5, year 3 is 3, year 4 is 2, year 5 is 6, year 6 is 5. The first four are already plotted. Plot year 5.

Now read it back

Looking at the plotted widths from that example sample, which year had the best growing conditions, and how do you know?

• Year 5, because its ring is the widest of the six
• Year 4, because the narrow ring shows the tree was conserving energy
• Year 6, because it is the most recent year on the graph
• Year 1, because the record starts from its width

How an ice core becomes a record

Put the process in order, from the ice sheet to a usable climate record.

• A long cylinder of ice is drilled out of an ice sheet and kept frozen
• The annual layers in the core are counted to date each depth
• Air bubbles trapped in each layer are extracted: that air is a sample of the atmosphere at the time
• The trapped gas and the chemistry of the ice are analysed for a temperature signal
• The results are assembled into a record of how conditions changed over time
• The record is compared with other proxies, so that no single line of evidence stands alone

Say it back

A colder period when ice sheets advanced is called a _____, and the warmer period between two of them is an _____. Evidence that was not made to record the climate but preserves a signal of it is called a _____ record. Tree rings give the finest detail because there is one ring per _____, but they only reach back a few thousand years. Ice cores reach back much further, so the choice between them depends on the _____ being asked.

Choosing your evidence

Three questions, three different jobs for the evidence. The trade-off between reach and resolution decides each one.

• You need to know whether one particular summer four hundred years ago was unusually cold. Which evidence suits that question best?
• Now you need to know how conditions changed across a full glacial cycle, tens of thousands of years long. What changes?
• A classmate says historical sources are the best evidence because people actually wrote down what they saw. What is the honest reply?

How we know the old climate

Explain how we know what the climate was doing long before anyone was measuring it, and what the natural causes of those changes were.

• Explain what a proxy record is, and name at least three kinds
• For two of them, say how far back they reach and how precise they are
• Describe at least two natural causes of past climate change and how each one works
• Make clear which causes are natural and which are human, and keep them separate
• Finish with the reason no single line of evidence is enough on its own