Reacción a "Reactions: skeletons of marine sponges suggest that global temperature has already exceeded the 1.5°C limit"

If we are to meet climate targets, we obviously must agree internationally on what these targets are to be. To do this, we need a robust and agreed definition of pre-industrial baseline that all temperature anomalies are to be measured against. This paper attempts to do just this by providing a high-resolution temperature record from sponge skeletons collected in a region that could be argued to reflect global change. These archives reveal that the current warming trend may have started earlier, and may have already reached 2°C above pre-industrial levels – a potentially significant benchmark given UN climate targets.

“However, we must keep in mind some considerable complexities surrounding the use of sponge skeleton chemistry as archives of past ocean change. We need to know far more about how these animals make their skeletons, and exactly how their chemistry relates to ambient temperatures – something we don’t have a good handle on now. We need a better understanding of these proxies before we can use them to make important statements about the state of the climate with confidence.

“This study calibrates their temperature archive by comparing sponge chemistry to sea surface temperature (SST) records from the latter part of the twentieth century, and they find a much greater temperature sensitivity than previous studies. Whilst this sensitivity is potentially very useful when building SST anomaly archives, we really need to delve into this more to understand it better. The calibration here only spans a portion of the temperature anomalies since pre-industrial times – it’s very important to know how the chemical proxy ‘works’ across a larger range. Experiments with sponges grown under controlled temperature conditions could be very useful in this respect, if challenging given their slow growth rates, as well as more field studies.

“There’s clearly more we need to know about these sponge archives, and we need more high-resolution climate records from more locations to build a robust, global picture. However, the importance of this paper is that it makes us ask the question: what if the planet has already warmed more than we thought?