Imagine you are considering what meals to have throughout the week, best-before and use-by dates make it a lot easier to know when your food is going to be fresh or not, so you can plan accordingly. This can be scaled up to thinking about climate change. We do not have the knowledge of a use by date when worrying about climate change though, we only have very vague ideas and predictions about when the Earth's climate system is going to deteriorate beyond repair. Climate scientists are striving to improve climate models every year so we can better understand the climate's timescale. However, there are so many factors and values that climatic variables are regularly simplified to rudimentary scales and models. Today we are going to look at how climate researchers monitor past, current, and future climates, and the limitations behind these methods.
Climate Change Data Collection
There are a variety of variables that are affected by altering climates. These include atmospheric carbon dioxide, temperatures, cloud cover, precipitation, sea levels, and ice cover. With modern advances in measuring technology, we are able to obtain current measurements for these variables, but first, let's look at how climate scientists measure past climates.
Paleoclimatology
Paleoclimatology is the study of past climates, with preceded the introduction of global instrumental records (1880).
Modern techniques of measuring climatic conditions only date to the 1880s, so climate proxies are important in providing accurate reconstructions of the Earth’s climate over the course of history.
A climate proxy is an observable characteristic of preserved matter that can be analysed and compared with similar preserved material to deduce the era from which this preserved matter originated.
Analysis of the chemical properties and comparable physical characteristics of this proxy can provide an insight into the climatic conditions of the period. Here are some examples of climate proxies:
Boreholes
Boreholes are narrow shafts that extend deep into the ground. By measuring the temperature at various depths down the borehole and inputting the data into an inverting mathematical formula, surface temperature values can be estimated. This is possible because of the slow propagation of heat downwards through the ground once the surface has been warmed. Therefore, measuring at different depths of the borehole can provide us with the information to reconstruct surface temperatures of past time periods.
Borehole depth measurements do not need to be calibrated like other climate proxies, as they are already a temperature. Other climate proxies are often different measurements like length/density of a tree ring or isotope concentration in an ice core.
Tree Rings
Dendroclimatology is the reconstructing of past climates through the study of tree rings.
Measurement of the width and density of these rings, coupled with analysis of their chemical properties provide invaluable information on the climatic conditions that impact tree growth. These conditions include:
- Temperature
- Rainfall
- Solar output
- Soil fertility.
During calibration of tree ring data, growth measurements undergo rigorous statistical analysis to find the correct model to convert them into climatic data. This tree ring data as well as monthly climatic data, like temperature and precipitation, are used to deduce which months are involved with tree growth the most. Measurements are often taken in the months of the growing season. When conditions are favourable, tree rings are wider.
Out of all the climate proxies, tree rings are the most easily accessible and observable because of the abundance of trees on the planet and their annual growth rings.
Figure 2: tree ring
Ice Cores
Ice cores are cylindrical samples of ice resulting from the compression of snow over time by new layers of snowfall falling on the surface. Measuring the concentrations of greenhouse gases (such as carbon dioxide and methane) of these air bubbles can give us an idea of ancient atmospheric conditions, and therefore the relative atmospheric global warming. Ice cores excavated by large drills from polar regions often exhibit a layered structure near the top of the ice core. These layers can represent annual climates, with the lighter layers representing clean snow that falls because of increased moisture in the air resulting from increased global temperatures, and the darker layer corresponding to cold, winter seasons with little snowfall and strong winds transporting dust from elsewhere which gets trapped in the ice.
Figure 3: ice core extracted using a drill
Analysis of the proportions of the oxygen isotopes O-16 and O-18 in the water molecules in the ice core can give us information about the climatic conditions of the time. The lighter O-16 makes up 99% of naturally occurring oxygen on earth. Evaporation of ocean water and consequent precipitation in the global water cycle leads to ocean water being present in preserved ice cores. During glacial periods, O-16 molecules become trapped in glacial ice, so the oceans gain more O-18 isotopes, so these periods are easily determined. Warmer water contains more of the O-16 isotope as it evaporates quicker than the heavier O-18 but will condense and fall as precipitation at a slower rate than water containing more O-18 isotopes.
The resolution power of ice cores decreases as you measure further down the core, as the compression is so great you cannot distinguish between layers.
Current Methods of Monitoring Climate
Modern technological advances allow climate scientists to measure the current state of the climate fairly accurately. Here are some of the methods they use:
Global Mean Surface Temperatures (GMST)
Climate change can often be attributed to changing global temperatures, so a comprehensive set of global measurements is essential in understanding current climate patterns. Climate scientists will combine temperature measurements from the air just above land masses, taken by ships and buoys, and taken by extra-terrestrial satellites. These measurements are compared to 30-year averages, and temperatures greater than the average are recorded as positive anomalies while temperatures colder than the average are negative anomalies.
30-year averages are used because we would expect the Earth's climate to vary naturally between decades because of a variety of phenomena like volcanic activity, ocean cycles, and orbital cycles.
Carbon Dioxide and other Greenhouse Gases
Greenhouse gases warm the Earth by contributing to the greenhouse effect, so measuring trends in their concentrations in the atmosphere helps climate scientists to attribute recent global warming. Since pre-Industrial times (around 1750) atmospheric carbon dioxide levels have increased by 50%. These current levels are unprecedented compared to data collected from climate proxies and are posing the greatest threat to the climate system.
The greenhouse effect is when infrared radiation re-emitted from the Earth's surface is reflected back towards the surface by atmospheric greenhouse gases.
Melting Sea Ice
Measurements from Arctic and Antarctic research laboratories and satellite images have shown substantial retreats in sea ice over the last 50 years. Melting sea ice can be attributed to rising global temperatures. In the Arctic, we are currently witnessing a 13% loss in sea ice per year.
Rising sea levels
Melting ice sheets and rising sea levels are intrinsically linked in the fact that melting ice adds millions of gallons of water to the oceans every year. Rising temperatures will directly affect sea levels too. Expanding water molecules from increasing temperatures around the equator will spread out and cause sea levels to rise around the world. Global sea levels have risen 8-9 inches around the world since 1880.
Precipitation and storms
Rainfall and storm frequency is linked to global warming because rising temperatures are intensifying the global water cycle. This means that evaporation is occurring at a faster rate, and clouds are condensing and forming precipitation faster too. This is resulting in more frequent rainfall and storms and increased severity of these events.
Climate Models
Climate models are computer simulators that project climate patterns over time. To do this, they accumulate reliable and consistently taken data over a certain time period and deduce how changing atmospheric conditions have altered the climate. Once the effects of changing atmospheric conditions have been calculated (such as the addition of greenhouse gases or aerosols) researchers can input various potential values from human activity. The results of the simulation will show the impact of varying levels of human activity on different climatic conditions.
The effect of atmospheric changes on climate is called climate sensitivity.
Here are some groups that monitor climate change:
- National Oceanic and Atmospheric Administration (NOAA)
- Met Office (weather)
- Department of Energy and Climate Change (DECC)
Difficulties Monitoring Climate Change
These methods are not all completely sound. There are many limitations and complications which need to be taken into account. Let's have a look at some of the difficulties climate researchers face when monitoring climate change:
Past Climate Change
Here are some of the limitations associated with climate proxies:
- Boreholes: When using boreholes to produce temperature measurements for recent times (20th century onwards) the resolution is only a few decades but when measuring lower depths for older temperatures the resolution decreases (between centuries instead). This decrease in resolution is a consequence of the thermal smearing that acts on the heat transferring downwards from the surface. Additionally, groundwater can run into boreholes and cause a decrease in temperature at the points being measured.
- Ice cores: the resolution of an ice core decreases as you measure deeper and the ice is highly compressed. They show limited sensitivity during glacial periods too.
- Tree rings: many climatic and non-climatic factors affect tree ring measurements. Sunlight, rainfall, temperature, and wind are all climatic factors. Examples of non-climatic factors are age, gene pool, competition, human impact, herbivore presence, disease, CO2 concentration, insect damage and natural disasters. These variables are known as 'confounding factors.
To distinguish between the confounding factors associated with tree rings researchers can use ‘limiting stands’, which are points on the tree where the tree is only affected by a certain factor up to a certain point. When you measure past this point, the factor is limited, and other factors can be measured in isolation.
To distinguish between the confounding factors associated with tree rings researchers can use ‘limiting stands’, which are points on the tree where the tree is only affected by a certain factor up to a certain point. When you measure past this point, the factor is limited, and other factors can be measured in isolation.
Difficulties predicting climate change
Projecting future changes in climatic conditions is extremely difficult too:
- Anomalies and off-measurements: there are an unbelievable amount of measurements that are inputted into climate model calculations. Any slight mistakes in these measurements could result in subtle changes in how the model works, and eventually lead to very different values.
- Cloud cover: there is a lot of uncertainty when modelling the effects of clouds on incoming infrared radiation and climate. Clouds form such a random and vast pattern in the atmosphere that they are basically impossible to model correctly.
- Energy from the sun: infrared radiation from the sun has a tremendous effect on the temperature of the Earth, far more than greenhouse gases. Therefore, even small miscalculations in the energy output of the sun will result in climate models being thrown off completely.
Difficulties monitoring and predicting climate change - Key takeaways
- Climate proxies are preserved material that can be analysed and compared to deduce the climatic conditions in past eras. Examples of climate proxies are ice cores, boreholes, and tree rings.
- Current climates are a lot easier to measure because of the wide range of technologies we have access to. Measurements of temperature, atmospheric carbon dioxide, sea levels, and rainfall are methods of quantifying climate change.
- Future climates are predicted through the use of climate models. Large data sets are combined and climate simulators are produced from them. Researchers can input various levels of greenhouse gases into the simulator and see the outcome.
- However, there are limitations in climate modelling. There are so many measurements and calculations that the smallest error can be costly.
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