What you'll learn
- How to distinguish weather, climate, global warming and climate change.
- How scientists use direct measurements and proxy evidence to reconstruct climate patterns.
- Why climate models are useful, but always include uncertainty.
- How climate change can affect species distributions, food webs and biodiversity.
Climate, weather and the greenhouse effect
Weather, climate and climate change
Weather is the short-term state of the atmosphere, such as today’s temperature, rainfall and wind. Climate is the long-term pattern of weather in a region, usually considered over at least 30 years. Climate change is a sustained change in climate averages, ranges or extremes over time.
A single hot day is not climate change. A long-term increase in mean global temperature, changes in rainfall patterns, melting ice and sea-level rise together provide stronger evidence.
Global warming is the increase in Earth’s mean surface temperature. It is one part of climate change, because climate change also includes effects such as altered rainfall, more frequent heatwaves and changing ocean conditions.
The greenhouse effect is a natural process. The Earth absorbs short-wave radiation from the Sun and emits long-wave infrared radiation. Greenhouse gases absorb some of this outgoing infrared radiation and re-radiate it, including back towards Earth’s surface.
Important greenhouse gases include:
- carbon dioxide, CO₂
- methane, CH₄
- water vapour, H₂O
- nitrous oxide, N₂O
Anthropogenic means caused by human activity. Burning fossil fuels, deforestation and some agricultural practices increase greenhouse gas concentrations, causing an enhanced greenhouse effect.
Natural versus enhanced greenhouse effect
The natural greenhouse effect keeps Earth warm enough for life. The enhanced greenhouse effect is the extra warming caused when human activities increase greenhouse gas concentrations.
Ozone layer confusion
Damage to the ozone layer is not the main cause of global warming. Climate change is mainly linked to increased greenhouse gases absorbing outgoing infrared radiation.
Evidence for climate change
Scientists do not rely on one graph or one measurement. They use many independent sources of evidence that point in the same direction.
Direct evidence
Direct evidence comes from measurements made using instruments. Examples include:
- thermometer records of air and sea-surface temperature
- satellite measurements of ice cover, sea level and radiation
- atmospheric carbon dioxide measurements
- glacier retreat and changes in Arctic sea ice
- recorded changes in flowering times, migration and breeding seasons
Modern records show that global mean temperature has increased by over 1 °C since the late nineteenth century. The exact value depends on the dataset and reference period used, so you should describe the trend rather than memorising a single number.
Proxy evidence
Proxy evidence
A proxy is an indirect source of evidence used to infer past climate. Proxy evidence is especially useful for studying climates from before humans had thermometers and satellites.
Common climate proxies include:
- Ice cores: cylinders of ancient ice drilled from ice sheets. Trapped air bubbles show past atmospheric CO₂ and CH₄ concentrations. Isotope ratios in the ice can also indicate past temperatures.
- Pollen grains: pollen has a resistant outer coat and can be preserved in lake or peat sediments. Different plant species grow in different climates, so fossil pollen gives evidence about past vegetation and climate.
- Tree rings: wider rings often indicate better growing conditions, such as warmer temperatures or more water, although interpretation depends on the species and habitat.
- Marine sediments: shells of microscopic organisms can contain chemical signatures linked to past ocean temperature and ice volume.
Interpreting pollen in sediment
A sediment layer contains lots of grass and birch pollen, but very little oak or lime pollen. Modern grassland and birch woodland are common in cooler conditions, while oak and lime woodland are more common in warmer temperate conditions.
- Compare the fossil pollen types with the known climate preferences of modern plants.
- Grass and birch suggest open or cooler habitats, while low oak and lime suggests warmer woodland was less common.
- Infer that the climate when that sediment layer formed was probably cooler than a layer dominated by oak and lime pollen.
Strong evidence comes from agreement
Ice cores, instrumental records, tree rings, pollen and satellite data have different weaknesses. If independent methods show the same long-term trend, the conclusion is more reliable.
Quantitative climate data
Climate graphs often show a temperature anomaly rather than raw temperature.
Temperature anomaly
A temperature anomaly is the difference between a measured temperature and a chosen reference-period mean. It shows whether a year was warmer or cooler than the baseline.
Using anomalies makes it easier to compare data from different locations, because a cold region and a warm region can both be compared with their own usual climate.
You may also see moving averages, where several neighbouring values are averaged to smooth short-term fluctuations. This helps reveal the long-term trend.
Calculating a rate of warming
A climate dataset shows a temperature anomaly of -0.1 °C in 1910 and +1.1 °C in 2020. Calculate the average rate of warming per decade.
- Calculate the change in temperature anomaly:
- Calculate the time interval:
- Divide the temperature change by the time interval:
- Round sensibly:
Weather is not climate
Do not use one unusually cold winter to argue against climate change. Climate conclusions are based on long-term patterns, not isolated weather events.
Climate models
Climate model
A climate model is a computer simulation that uses mathematical equations to represent parts of the Earth system, including the atmosphere, oceans, ice, land and carbon cycle.
Models divide the Earth into grid cells and calculate how energy, water and gases move between them. Inputs can include greenhouse gas concentrations, solar radiation, volcanic eruptions, aerosols, land use and ocean circulation.
A scenario is a possible future pathway, such as high emissions or reduced emissions. A scenario is not a guaranteed prediction; it depends on human decisions and policies.
Testing models
Models are tested using hindcasting: scientists give the model past conditions and see whether it can reproduce climate patterns that actually happened. If a model cannot explain past climate, scientists have less confidence in its future projections.
Models are usually run many times as an ensemble. An ensemble is a group of model runs, often using slightly different assumptions or starting conditions. This gives a range of possible outcomes.
Testing models against observed warming
Observed warming over a period is 1.2 °C. A model using only natural factors predicts 0.2 °C warming. A model using natural factors plus human greenhouse gas emissions predicts 1.1 °C warming.
- Calculate the difference between observed warming and the natural-only model:
- Calculate the difference between observed warming and the natural-plus-human model:
- Compare the differences. The natural-plus-human model is much closer to the observed warming.
- Conclude that this supports the role of anthropogenic greenhouse gas emissions, although real model evaluation uses many datasets and time periods.
Uncertainty does not mean guessing
A model projection can be uncertain but still useful. In biology and climate science, uncertainty usually means a calculated range of likely outcomes, not a lack of evidence.
Feedback in the climate system
Feedback
A feedback occurs when a change in one factor causes effects that influence the original change. Positive feedback amplifies the original change. Negative feedback reduces it.
An important example is the ice-albedo feedback. Albedo is the proportion of incoming radiation reflected by a surface. Ice has a high albedo, so it reflects lots of solar radiation. Dark ocean water has a lower albedo, so it absorbs more radiation.
If warming melts sea ice, more dark ocean is exposed. This increases absorption of solar radiation, causing more warming and further melting. That is positive feedback.
Other climate feedbacks include:
- thawing permafrost releasing methane, a greenhouse gas
- warmer air holding more water vapour, which can increase warming
- changes in plant growth affecting carbon dioxide uptake
- changes in cloud cover, which can either reflect sunlight or trap infrared radiation depending on cloud type and height
Feedbacks affect future change
Feedbacks are one reason climate projections are given as ranges. They can amplify or reduce warming, and their strength can vary between ecosystems and regions.
Effects on organisms and ecosystems
Climate change alters abiotic factors, meaning non-living environmental conditions such as temperature, rainfall, light intensity, salinity and oxygen concentration. These changes then affect organisms and food webs.
Species distributions
Species distribution
A species’ distribution is the geographical area where it is found. Distribution depends on abiotic factors and biotic factors such as competition, predation, disease and availability of food.
As temperatures rise, many species shift:
- towards the poles
- to higher altitudes
- into deeper or cooler water
- into shaded or wetter microhabitats
However, movement is not always possible. Species may be limited by mountains, roads, farmland, oceans, lack of suitable soil, lack of mutualistic species, or poor dispersal ability.
Estimating an uphill range shift
A mountain plant grows best at a mean summer temperature of 12 °C. Climate warming raises the temperature at its current altitude by 1.3 °C. Use a temperature decrease of 6.5 °C per 1000 m increase in altitude to estimate how much higher the plant may need to grow.
- The plant needs to move to an altitude where the temperature is about 1.3 °C cooler.
- Set up the proportion using the given temperature decrease:
- Calculate the altitude increase:
- The plant may need to grow about 200 m higher, if suitable habitat exists there.
Simple range estimates have limits
Altitude and latitude estimates ignore factors such as rainfall, soil type, competition, seed dispersal and habitat fragmentation. Use them as rough predictions, not exact forecasts.
Timing and food webs
Phenology is the timing of seasonal biological events, such as flowering, leaf fall, migration, egg laying and hibernation. Climate change can shift these timings.
A trophic mismatch occurs when interacting species respond differently. For example, caterpillars may hatch earlier because spring temperatures rise, but bird migration may not shift enough. Chicks may then hatch after peak caterpillar abundance, reducing survival.
Biodiversity and extinction risk
Biodiversity means the variety of living organisms. It includes genetic diversity, species diversity and ecosystem diversity.
Climate change can reduce biodiversity if:
- populations cannot migrate quickly enough
- habitats become smaller or more fragmented
- extreme events such as droughts, floods or heatwaves increase mortality
- specialist species lose the specific conditions they require
- invasive species, pests or pathogens spread into new areas
Natural selection may help some populations adapt, but only if there is genetic variation and enough time. Rapid climate change can outpace adaptation, especially in species with long generation times or small populations.
Oceans and freshwater
Warmer water holds less dissolved oxygen, which can stress aquatic organisms. Increased atmospheric CO₂ also dissolves into seawater, causing ocean acidification. This reduces carbonate ion availability, making it harder for corals and shell-forming organisms to build calcium carbonate structures.
Coral reefs are also threatened by coral bleaching, where corals lose their symbiotic photosynthetic algae during heat stress. This reduces the coral’s energy supply and can lead to death if stressful conditions continue.
In the exam
- Separate evidence, mechanism and effect: what shows climate is changing, why it is changing, and what it does to organisms.
- Use long-term trends and multiple data sources; avoid conclusions based on one year or one location.
- When evaluating models, mention inputs, assumptions, validation against past data, uncertainty and emission scenarios.
- For ecosystem effects, link the abiotic change to survival, reproduction, distribution, food webs and biodiversity.
Check yourself
- Why are ice cores and pollen grains described as proxy evidence?
- How does the enhanced greenhouse effect differ from the natural greenhouse effect?
- Why might two interacting species become mismatched as the climate warms?
