Earth-observing satellites have been orbiting our planet since the late 1970s. Some carry instruments that measure the microwave energy given off by the atmosphere, from which scientists can estimate temperatures in different atmospheric layers. The initial analysis of the satellite data showed no significant warming trend in the troposphere, contradicting both surface measurements and radiosondes. But these initial calculations didn’t correctly account for the fact that satellite orbits had gradually drifted, leading to measurements being taken slightly later each day. Once these errors were recognised and corrected, satellite data showed a warming of between 0.1 and 0.2 °C per decade. This is in broad agreement with surface and radiosonde measurements.

Filled with hydrogen or helium, weather balloons carrying scientific instruments called radiosondes soar up to 20 km into the atmosphere – higher than most aeroplanes fly. The radiosonde, which usually weighs less than half a kilogram, measures temperature as it rises through the atmosphere, beaming its data back to laboratories. The radiosondes eventually fall back to Earth, but are rarely recovered. Scientists record their data and combine the individual measurements into a global average temperature. Beginning in 1958, radiosonde data show a warming trend of between 0.1 and 0.2 °C per decade. This is in broad agreement with satellite and surface measurements.

All objects give off energy – the Sun, the Earth, even the human body. The intensity and frequency of the energy depends on the object’s temperature: the hotter the object, the higher the energy’s average frequency and intensity. Because temperature varies throughout the Earth’s atmosphere, different areas give off energy at different frequencies and intensities. So by measuring the energy given off by different atmospheric layers, scientists can estimate their temperatures. Satellites measure the intensity of microwave energy from the lower troposphere, the mid-to-upper troposphere and the lower stratosphere. These measurements began in 1978, enabling scientists to estimate variations in temperature in these layers over the past 30 years.

Climate models simulating the Earth’s atmosphere predict that when the world warms up, the upper troposphere should warm faster than the surface, especially in the tropics. But initial weather balloon and satellite measurements suggested that the upper troposphere was hardly warming at all. Some measurements even showed a slight cooling trend. Some scientists thought there might be a problem within the climate models, causing them to simulate the atmosphere incorrectly. But later measurements taken over longer time periods show a warming trend in the upper troposphere after all. There are still many uncertainties in the measurements, but the most recent analyses are in broad agreement with climate model predictions.

In contrast to the warming trends observed by satellites and weather balloons in the troposphere, both types of measurement have recorded a marked cooling trend in the stratosphere. Though this might seem strange, a cooling stratosphere is actually consistent with what scientists expect from global warming caused by increased greenhouse gas levels. Stratospheric temperature depends partly on the amount of energy it receives from the troposphere below it. Human activities are emitting greenhouse gases, trapping more heat energy in the lower atmosphere, causing the stratosphere to cool down while the troposphere warms up. Ozone depletion, caused by human emissions of chlorofluorocarbon (CFC) gases has also played a part.

The world’s oceans are a vast reservoir of energy. Because water warms more slowly than air, the rise in sea surface temperature is slightly slower than the corresponding rise in surface air temperature. But there has been a marked increase in the amount of heat energy stored in the oceans. Temperature probes take measurements from the surface of the sea to depths of several hundred metres below. Scientists have compared probe measurements with satellite data to calculate changes in the heat energy contained in the ocean’s surface layer. The calculations indicate that more than 80% of the increased heat energy in the climate has been taken in by the oceans.

• Met Office: Using Weather Balloons
• NASA: TERRA Satellite Mission