The impact of updating World Meteorological Organization baselines on Australian climate change messaging

Baselines matter in our perception of which is the climate ‘normal’ and what benchmark we use to track climate changes. Use of recent baselines grounds expectations about historical and future climates in recent lived experience, but differences between even similar recent periods shape our perceptions and interpretations of climate information. Here, we examine various examples of the effect of changing standard World Meteorological Organisation baselines (1961–90 and 1991–2020), as well as the pre-industrial baseline of 1850–1900, in the context of climate change messaging in Australia. We find the choice of baseline notably affects the messaging, with different connotations in two different contexts. The first context is where there are ongoing and attributable trends, such as in temperature, sea level and some localised precipitation changes. Here, the baseline grounds our perception of ‘normal’ to a particular point in an ongoing trend, and this normal is increasingly different from current lived experience. Also, in this context, reported ranges of change are smaller for more recent baselines. The second context is where there are weak climate change trends, and differences between baselines are primarily due to climate variability, such as most regional precipitation changes. Here, the baseline also grounds the perception of the climate ‘normal’, not as a particular point on an ongoing trend but rather in a particular state of climate variability. Here, a challenge can be to convey plausible future changes due to climate change where there is currently only a weak signal overwhelmed by variability. This study demonstrates the importance of baselines in climate change messaging and gives a simple framework for considering some key questions such as signal and variability.