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Key Points
- 2023 is breaking temperature records, with early July recording the highest planetary average temperatures.
- Global warming has been on an upward trend since the mid-1970s, with each decade hotter than the last.
- The rise in temperature follows a step-like progression, with significant increases during El Niño events.
Now, in 2023, all kinds of records are being broken. The highest daily temperatures ever recorded globally occurred in early July, alongside the largest sea surface temperature anomaly ever.
But global mean surface temperature does not continue relentlessly upwards. The biggest increases, and warmest years, tend to happen in the latter stages of an El Niño event.
Atmospheric carbon dioxide concentrations continue to climb relentlessly upwards despite the Paris Agreement and the many countries and organisations (cities, companies) that have made good on their commitments to cut emissions.
The major El Niño event in 2015-16 changed that. 2015 became the warmest year on record, ending the hiatus, only to be surpassed by 2016, which remains the warmest calendar year so far in many records.
With the Pacific Decadal Oscillation, there were major changes in Pacific trade winds, sea-level pressure, sea level, rainfall and storm locations throughout the Pacific and Pacific-rim countries. These changes extended into the southern oceans and across the Arctic into the Atlantic.
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Accordingly, during the positive phase of the Pacific Decadal Oscillation, more heat is deposited in the upper 300 metres of the ocean, where it can influence global temperatures. During the negative phase, more heat is dumped below 300 metres, contributing to the overall warming of the oceans but lost to the surface.
Sea-level rise comes from both the expansion of the ocean as it warms and the melting of land-based ice (glaciers and ice sheets in Greenland and Antarctica). This puts more water into the oceans. Fluctuations occur as rainfall is partitioned differently between land and the ocean, with more rain on land during La Niña events.
This set the stage for June to have record high surface air temperatures globally. In early July, they hit the highest values on record.
Kevin Trenberth is a highly regarded climate scientist and Distinguished Scholar at the United States’ National Centre of Atmospheric Research. He is also Affiliate Faculty at the University of Auckland.
