Effects of climate change and modellingEdexcel International A Level Biology: Revision notes

Scientists use models to predict how climate will change. A simple model fits a trend line to data and extrapolates it, which means extending the line beyond the measured range. Computer models are more complex: they include the physics and biology of the atmosphere, oceans, ice and vegetation, and are run with different assumptions about future greenhouse gas emissions.

Limitations: extrapolation assumes the pattern will continue, but feedback processes can change the rate. Positive feedback speeds warming, for example melting ice exposes darker surfaces that absorb more radiation, and thawing soils release methane. Negative feedback slows it, for example more cloud cover or faster plant growth. Future emissions depend on human choices, natural events such as volcanic eruptions are unpredictable, and the data used have limits in range and accuracy. The further beyond the data a prediction goes, the less reliable it is, so predictions are given as a range with an uncertainty rather than as a single value.

A warmer atmosphere holds more water vapour and more water evaporates from the oceans, so rainfall patterns change: some regions become wetter, with more intense storms and flooding, and others become drier, with longer droughts. Changes in ocean currents and atmospheric circulation shift where rain falls.

Seasonal cycles shift. Spring starts earlier and autumn later, so the growing season is longer in temperate regions, and the timing of events such as leaf opening, flowering, insect emergence, migration and breeding changes. These changes matter because many organisms time their activity to cues such as temperature or day length.

The distribution of a species depends on its tolerance of temperature and water availability. As temperatures rise, species with narrow tolerance ranges may be restricted: cold-adapted species move to higher latitudes or altitudes, and warm-adapted species spread into regions that were previously too cold, including pests and the vectors of disease. Species that cannot disperse fast enough, or that are already at the top of a mountain, may die out locally.

Life cycles are affected because development rates depend on temperature. Insects may emerge earlier, plants flower earlier, and some species can complete more generations in a year. Species respond at different rates, so timings can become mismatched, for example if the peak abundance of caterpillars comes before the chicks of a bird that feeds on them hatch. A mismatch reduces food for the young, lowers breeding success and can reduce population size.

As temperature rises towards the optimum, molecules have more kinetic energy, so enzymes and substrates collide more often and with enough energy, and the rate of reaction increases. Above the optimum, hydrogen bonds in the enzyme break, the tertiary structure and the shape of the active site change, and the enzyme denatures, so the rate falls.

The temperature coefficient Q10 measures how much the rate changes for a 10 °C rise: Q10 = rate at (T + 10) °C ÷ rate at T °C. A Q10 of 2 means the rate doubles. If the temperature interval is not 10 °C, use $Q_{10} = \left(\frac{R_2}{R_1}\right)^{10/(T_2-T_1)}$. Worked example: a rate of 18 units at 15 °C and 41 units at 25 °C gives Q10 = 41 ÷ 18 = 2.3. A Q10 below 1 means the rate has fallen, which is expected above the optimum, so Q10 applies only where the enzymes are not denaturing.

A typical investigation uses eggs, seeds or larvae of a small organism, such as brine shrimp eggs, incubated at a range of temperatures. The independent variable is temperature, set with thermostatically controlled water baths or incubators and checked with a thermometer. The dependent variable is a measure of development, such as the time for 50% of eggs to hatch, the percentage hatched after a fixed time, or the number reaching a stage.

Control variables include the number of organisms, their source and age, salinity or moisture, light, aeration and pH. Replicate each temperature (at least three) and calculate a mean. Use a range of temperatures that includes values above and below the optimum. Rate of development can be calculated as 1 ÷ time and used to find Q10. Handle live organisms with care and return or dispose of them humanely.