Evidence for Change

Eight hundred thousand years in a tube

At Dome C in Antarctica, a European drilling project pulled a core of ice 3,270 m long out of the ground. The deepest ice in it fell as snow around 800,000 years ago. That matters because snow does not settle cleanly: it traps air between the flakes, and as more snow piles on top the air is sealed into bubbles. Open a bubble from two metres down and you get air from the 1980s. Open one from three kilometres down and you get a sample of the atmosphere from before our species existed. Two measurements come out of a core like that. The trapped air can be analysed directly for carbon dioxide and methane. And the ice itself records temperature, because water containing heavier oxygen atoms evaporates slightly less readily in cold conditions, so the ratio of oxygen isotopes in each layer is a thermometer reading from the year the snow fell. Put them together and you get a paired record of temperature and greenhouse gas stretching back eight hundred millennia. What it shows is a world that has swung repeatedly between cold glacial periods and warmer interglacials, with carbon dioxide rising and falling alongside temperature in a band of roughly 180 to 280 parts per million. It never left that band, not once in 800,000 years. Today it is over 420. This module is about the evidence for changes like that, natural and recent, and about how you tell which kind of cause you are looking at.

Five terms, each with what produced it

The specification note for this element asks for named evidence rather than vague appeals to science, so each of these carries the record it comes from and the span it covers.

Order how ice becomes a record

These six stages turn falling snow into a measurement of an ancient atmosphere. Put them in order.

• Snow falls on the ice sheet and settles loosely, with air between the flakes
• Further snowfall buries it, and the weight above compresses it into ice
• The air between the flakes is sealed off into bubbles as the ice closes around it
• A core is drilled out, with the deepest ice being the oldest
• The annual layers are counted and dated down the length of the core
• Trapped air is analysed for greenhouse gases and the ice itself for its oxygen isotope ratio

How does one core give two readings?

An ice core yields both a carbon dioxide record and a temperature record. Which answer explains how?

• They come from two different parts of the same ice. The air bubbles sealed into the ice as it compacted are physical samples of the ancient atmosphere, so their carbon dioxide and methane can be measured directly. The temperature comes from the ice itself: water made with heavier oxygen atoms evaporates slightly less readily in colder conditions, so the ratio of oxygen isotopes in each annual layer indicates how warm it was when that snow formed. Dating the layers lets the two be read against each other year by year.
• The colour of each layer shows how warm the year was.
• Thicker layers mean warmer years, and the gas is calculated from the thickness.
• Scientists estimate both from computer models of the past.

Natural causes against human causes

Both columns are real, and an answer that denies the first has not understood the topic. What decides the argument about recent warming is not whether natural causes exist but how fast each kind of cause operates.

Plot the instrumental record

The axis across the bottom counts twenty-year steps from 1900, so 0 is 1900, 3 is 1960 and 6 is 2020. The axis up the side shows how much warmer the global average was than in 1900, measured in tenths of a degree Celsius. The points for 1900 and 1960 are already marked. By 2020 the global average was about 1.2 degrees Celsius warmer than in 1900. Plot that point.

Match each record to what it can tell you

Name the evidence, and mind the gap between weather and cause

The specification note for this element is blunt about what loses marks: vague appeals to what scientists believe. An answer that says researchers have found the planet is warming has told the examiner nothing. Name the record, say what it physically is, and say what it can reach. That is the whole difference between an answer in the bottom band and one in the top. Each kind of evidence has a reach and a weakness, and pairing them is what makes the case strong. Instruments are the most accurate and the shortest in span. Tree rings give you a single year at a time but only where trees grow and only for a few thousand years. Historical records such as harvest dates, ships' logs and paintings of frozen rivers stretch the picture back further but were made by people recording something else entirely. Ice cores reach furthest of all and sample the atmosphere itself, but they come from the poles and they cannot resolve a single summer. No one record proves anything; they corroborate one another, and saying so is worth a sentence. On causes, the trap is to treat natural and human explanations as rival claims where you must pick a side. They are not. Orbital cycles, volcanic eruptions and solar variation are all real and all documented, and they explain the swings visible across the long record. The argument about the recent change turns on rate, not on whether natural causes exist: the slow causes work over tens of thousands of years, the volcanic ones reverse within a few, and none of them can account for a change of this size compressed into a couple of centuries. Finally, watch the command word. "Describe the evidence" wants the records and what they show, with no argument attached. "Explain the causes" wants mechanisms. "Assess" wants you to weigh and then decide.

Complete the evidence facts

The EPICA ice core from Dome C in Antarctica runs 3,270 m deep and holds around _____ years of record, because air is sealed into _____ as the snow is compressed, giving a direct sample of the ancient atmosphere. Across that whole span carbon dioxide stayed between roughly 180 and 280 parts per million; it now stands above _____. Tree rings add a different strength, since a tree lays down one ring each year and the _____ of that ring records how good the growing season was. The longest unbroken instrumental record in the world is the Central England Temperature series, which begins in _____.

Spot the three sound statements

Select the THREE statements that are true.

• Natural causes are real and well documented, but the argument about recent warming turns on rate: orbital cycles operate over tens of thousands of years and volcanic cooling reverses within a few, so neither has the pace to produce what the last two hundred years have shown.
• Large eruptions cool the planet temporarily rather than warming it, because ash and sulphur dioxide in the upper atmosphere reflect sunlight; Pinatubo in 1991 lowered the global average by roughly half a degree Celsius for about two years.
• The strength of the case comes from different records agreeing: ice cores reach furthest, tree rings give annual precision, instruments give accuracy, and historical sources bridge the gap between them.
• Climate did not change at all before humans began burning fossil fuels.
• Ice-core carbon dioxide figures are estimates produced by computer models.
• Volcanic eruptions are a major cause of long-term global warming.

A 6-mark evidence answer, marked

Describe the evidence that climate has changed over time. [6 marks] The furthest-reaching evidence comes from ice cores. The EPICA core drilled at Dome C in Antarctica is 3,270 m long and covers roughly 800,000 years, and because air becomes sealed into bubbles as falling snow is compressed into ice, each layer holds a physical sample of the atmosphere from the year it fell. Measuring the carbon dioxide in those bubbles shows it cycling between about 180 and 280 parts per million across the whole period, while the ratio of oxygen isotopes in the ice itself indicates the temperature at the time, so the two can be read against one another. Tree rings provide a shorter record at much finer resolution, since a tree adds one ring a year and the width of each reflects the growing conditions, and by overlapping rings from living trees with ancient timber, continuous chronologies have been built back thousands of years. Historical sources extend the picture where instruments do not exist: harvest records, ships' logs and paintings of fairs held on a frozen Thames, which last froze completely in 1814, all indicate colder winters in the past. Finally, instrumental records give direct measurement, with the Central England Temperature series running unbroken since 1659, global records from 1850 and satellite coverage since 1979. Each record has a different reach and a different precision, and the case is strong because they agree with one another. What earned the marks: four distinct types of evidence, each named specifically rather than gestured at; what each one physically is and how it records climate; the span each covers; and a closing observation about corroboration. Note that the answer describes and does not argue, because the command word was "describe".

Find the two that name their evidence

Five sentences from student answers. Three of them appeal vaguely to science. Tap the TWO that name a specific record and say what it shows.

• The EPICA core from Dome C covers around 800,000 years, and the air trapped in its bubbles shows carbon dioxide staying between roughly 180 and 280 parts per million throughout.
• Scientists have proved that the climate is changing.
• The Central England Temperature series has been recorded with instruments without a break since 1659, making it the longest such record anywhere.
• Everyone agrees that the weather is not what it used to be.
• Research shows that temperatures have risen over recent decades.

Build the rate argument

Assemble the sentence that settles the natural-versus-human question properly.

Four questions for the record

Each stage gives you an enquiry. Choose the evidence that can actually answer it.

• You want to know what the atmosphere contained a quarter of a million years ago.
• You need to know whether one particular summer eight hundred years ago was unusually cold, to the year.
• A large eruption has just thrown ash and sulphur dioxide high into the atmosphere. You are asked what it will do to global temperature.
• Someone argues that because the climate has always changed naturally, the recent warming needs no other explanation.

Explain the causes

Explain the natural and human causes of climate change. [9 marks]

• At least two of orbital change, volcanic activity, solar variation, with the timescale of each
• What physically changes about the energy arriving at or leaving the Earth
• The gases and where they come from
• The enhanced greenhouse effect, not just a list of sources
• The shift in carbon dioxide concentration and the period over which it happened