Understanding our changing climate, and the underlying causes of these changes, requires an understanding not just of changes at the surface of the Earth but throughout the atmospheric column. Furthermore, high-quality measurements are needed to separate the climate change signal from natural variability.
Water vapour is the most important greenhouse gas as it is responsible for about 60% of the natural greenhouse effect. There are vigorous discussions within the research community regarding whether stratospheric humidity has changed and whether any further change is expected to influence the Earth’s energy budget. At the same time, water vapour measurements, particularly around the tropopause, are afflicted with high measurement uncertainties. Even key mechanisms controlling humidity in this region are not fully understood, leading in turn to significant deficiencies in the predictive skill of global climate models. Currently, satellites and research-quality instruments on aircraft and balloon platforms are the main sources of humidity measurements around the tropopause. Differences between these measurement systems have been difficult to reconcile.
Existing records of upper-air temperatures are insufficient to meet the growing range of needs for studying climate. They greatly lack continuity, homogeneity and representativeness of data, because past measurements were seldom intended for climate research, but mainly for short-term weather forecasting. It is likely that similar problems will persist in the future. Therefore, a way of separating climate change signals from the inevitable non-climatic effects, caused by measurement biases, instrument instabilities and network inhomogeneities, is essential.