How we know about past climates
Written historical data and observations
Observations of weather and climate have been a vital tool for tens of thousands of years. Hunter-gatherers had to understand seasons in order to know when and where animals migrated. The advent of agriculture meant knowing when to plant and when to harvest. Sailing across water safely meant understanding the weather, ocean currents, and how this changed over months and year. Those observations were passed on to others others via in ships and farmers’ logs, travellers’ diaries, meteorological records (Fig. 3), and newspapers. These now form crucial written evidence of past climates and what factors, such as volcanic eruptions, contributed to short-term climate changes such as the ‘Little Ice Age’. A 650-year old hundred year record of the dates that grapes were harvested in Burgundy, France, for example, allowed scientists to reconstruct temperatures for that period.
Ice cores
In Video 1, Professor Richard Alley briefly explains how ice cores provide proxy data about past climates because ice has preserved the evidence in ice caps. He uses ice collected in New Zealand and Greenland to illustrate how bubbles of prehistoric air, which can be analysed for the amount of carbon dioxide (CO2) they contains, and ash from volcanic eruptions is preserved. The video was made in 2012, so it refers to the oldest ice core collected at that time as 400,000 years. In 2017, an ice core collected from Antarctica goes back 2.7 million years, extending our knowledge of past climates back to the beginning of the current Quaternary ice age.
Video 1: Ice cores
Tree rings (dendrochronology)
As a tree grows, it adds a new ‘ring’ around its trunk every year (Video 2). Some trees are hundreds to thousands of years old, so they contain annual records of climate for centuries to millennia. Because climate conditions have an effect on how a tree grows, when looked at closely, these rings also show what the growing conditions were like each year. When hundreds of tree rings are compared with one another, they reveal patterns of change over places as well as years. Combined with other proxy data such as pollen, this creates a clear picture of what the climate was like, how quickly it might have changed, and how quickly it’s changing today.
Video 2: Tree rings
Speleothems (stalactites and stalagmites)
It’s possible to use material that glaciers have deposited inside caves where speleothems (stalactites and stalagmites Fig. 4) form, to determine when glaciers advanced and retreated.
This has been done in New Zealand where a cave in Fiordland was repeatedly overrun by glaciers. Using uranium isotope techniques, the speletherms provided information as far back as 230,000 years.
Similarly, the patterns of growth of a flowstone speleotherm in Nettlebed cave beneath Mount Arthur, Kahurangi National Park (New Zealand’s deepest cave), have been able to shed light on the changes in vegetation and soil above the cave, going back 215,000 years.
Coral reefs
Corals extracted calcium carbonate from seawater (see the carbon cycle) to build hard skeletons. Like tree rings, they grow layers each year, and like tree’s they’re extremely sensitive to changes in climate conditions (Video 3). Each layer tells a story about the temperature and chemistry of the ocean that year, and some corals have been around for millions of years (only the surface layer is alive. The inner layers are dead).
Because different species of corals can grow only at certain depths, corals also show that when climate warmed in the past, sea levels rose abruptly when certain tipping points were reached, rather than gradually.
Video 3: Coral reefs
Sediment in oceans and lakes
Pollen is not the only thing that’s found in sediments on the ocean floor. Some species of single-celled aquatic dinoflagellates produce extremely strong cysts (Fig. 5). The distribution and quantity of this plankton is a signal for what the ocean and some lakes were like in the past.
Other small aquatic free-swimming plants, algae, small multi-celled organisms, and tiny shrimp-like ostracodes also provide evince of what the climate was like in the past.
Like the atmosphere, the chemistry of the ocean also changes with a changing climate. The changing proportion of oxygen isotopes in the ocean can be used to work out the age of water in the ocean.
Boreholes
Heat transfer through the ground is slow, so temperature measurements at different depths down a borehole when adjusted for the effect of rising heat from inside the Earth can measure the temperature of Earth’s surface as it was back in time. For example, at about 150m deep the temperature is the same as it was 100 years ago. The scale is not linnear, however, so the temperature at 500m is the same as it was 1,000 years ago.
The advantage of boreholes as climate proxies is that they don’t need to be calibrated; they are actual temperatures measured in real time. However, they record ancient temperatures as they were on the surface of the ground. Today, most weather observations are taken 1.5m above the surface, and this can be quite different under extreme conditions such as when snow is acting as a blanket. Groundwater might also affect the temperatures but this can be avoided by not using boreholes as proxies in wet area.
Figure 2 shows the variation in bore temperatues for different latitudes.
Pollen
Pollen is mostly microscopic, it’s everywhere, and thanks to a tough shell around it called sporopollenin, it’s practically indestructible (Video 4). Chemicals in some pollen older than 300 million years are so stable that they, and the story they tell about climate, can be compared to pollen today. This makes pollen an ideal climate proxy. The pollen from each plant has a unique shape, making it easy to work out the type of plant they came from. This tells us what kinds of plants grew where and how the climate changed over millions of years.
The video explains how pollen grains found in sediment in the ocean floor around Antarctica show that the continent was once covered in forests like today’s forests in Fiordland and the West Coast of the South Island.
Video 4: Pollen
Stomata in fossil (& modern) leaves
The number of stomata (Fig. 6) on leaves indicates the levels of carbon dioxide (CO2) in the atmosphere in the past. This is because plant leaves grow larger stomata when the there’s less CO2 in the atmosphere.
Transpiration is the process by which plants ‘exhale’ water vapour through their stomata. Plants lose more than 90% of their water through transpiration. However, in the last 150 years as CO2 has been increasing, the density of stomata in some plants has dropped 34%. This is restricting the amount of water vapour the plants release. This has implications for the water cycle, especially in tropical rainforests, which by definition create rain largely through transpiration.
Knowing this relationship between CO2 in the atmosphere and the stoma in leaves, the number and size of stoma in fossil leaves is a way to determine the amount of CO2 in the atmosphere during the period the year was dated through other methods such as radiometric dating.
Fossils and rocks (radiometric dating)
All living things are composed of elements, some of which have a molecular structure that decays over time; this is called its ‘half-life’. Video 5 explains how carbon in plants and animals is used to work out when they died.
Wikipedia has a comprehensive list of other elements used to date living tings and certain types of rocks are dated, going back to when Earth formed about 4.5 billion years ago.
Video 5: Radiometric dating
