Twenty years ago, the trend in annual mean global mean temperature became detectable. Ten years ago, regional seasonal mean temperature trends were becoming clear. Nowadays, we can see trends even in weather extremes. In this post I show trends in long-term meteorological station data for hot, cold and wet extremes, and share some thoughts on tropical cyclones and droughts. It is underpinned by a decade of studying these around the globe, often in collaboration with the WWA team. I make no claims for completeness: the world is large and complicated, and I simply do not know all the literature.
Guest post by Geert Jan van Oldenborgh
Continue reading Trends in extremes
The consequences of the Paris Agreement’s choice of the pre-industrial as its baseline have been discussed previously on this blog. This choice makes sense from a climate forcing perspective (as radiative forcings are measured with respect to a quasi-equilibrated state, and the well-observed recent past is not close to have finished responding to anthropogenic drivers). Looking back into the pre-industrial period, there are fewer instrumental observations of the temperature across the globe. So naturally our knowledge of the pre-industrial baseline temperature is uncertain.
Recent work, such as Hawkins et al. (2017) and Schurer et al. (2017), have looked to assess and quantify this uncertainty in light of future targets. The magnitude of this uncertainty, although small, becomes important when you consider the amount of warming left between today and the 1.5°C target. Continue reading Uncertainty in warming since pre-industrial times
In the Paris climate agreement it was agreed to limit global warming to well below 2 degrees Celsius and to drive efforts to limit the temperature increase even further to 1.5 degrees Celsius above pre-industrial levels. There has been a debate whether the ambitious climate change mitigation goal of 1.5 °C is still within reach. This was fueled by a paper by Millar et al (NGS, 2017), which claimed there was still a reasonable carbon budget left to reach this very ambitious goal. Others claim this is already physically impossible. We try to find out why they reach different conclusions. Two factors are important: definitions and cancellations of temperature trends after fossil CO2 emissions would cease. A similar discussion with other emphasis has been written by the authors of Millar et al. It is all about tenths of degrees. Continue reading Is the 1.5°C target still reachable?
The world has not seen a major volcanic event for at least 25 years, but the tropical volcano Mt Agung on Bali is now threatening to erupt. Mt Agung’s last eruption in 1963 was one of the largest during the 20th century and had widespread climatic effects. Going further back in time, ice core-based volcanic reconstructions reveal events – such as the Samalas eruption in 1257 in Indonesia – that were an order of magnitude larger than the 1963 eruption of Mt Agung.
Because the timing and intensity of eruptions cannot be predicted in advance, they are commonly excluded from projections of future climate. The imminent eruption of Mt Agung raises important questions: How feasible and important is the inclusion of potential volcanism in future climate projections? Will eruptions help mitigating long-term anthropogenic global warming? What is their effect on climate projection uncertainty? How do eruptions affect future climate variability and the likelihood of climate extremes? Continue reading What do future eruptions mean for climate projections?
Recent media headlines have again discussed the issue of whether climate models are overly sensitive to greenhouse gases. These headlines have misinterpreted a study by Millar et al. which was discussing carbon budgets to limit global temperature rise to 1.5°C above pre-industrial levels.
A recent study by Medhaug et al. analysed the issue of how the models have performed against recent observations at length and largely reconciled the issue. An overly simplistic comparison of simulated global temperatures and observations might suggest that the models were warming too much, but this would be wrong for a number of reasons. Continue reading Are the models “running too hot”?
For twenty years between 1883 and 1904, three intrepid weathermen lived at the top of Ben Nevis – the highest mountain in the UK – experiencing some of the worst weather the country has to offer.
Every hour, day and night, winter and summer, and whatever the weather, one of them would step outside and check the meteorological instruments, diligently recording the observations.
This was a uniquely Victorian-era endeavour. Science for the sake of science. Rather than exploring the world’s polar regions like some of their contemporaries, these Weathermen were exploring the atmosphere.
There was simply no other way of learning in detail about how the atmosphere changed with height without living at the top of a mountain. So that is what they did.
Continue reading Weathermen of Ben Nevis
As the annual September sea ice minimum in the Arctic approaches, the usual questions arise about whether this year will set a new record for the extent or volume of ice left at the end of the summer. Although there was a new winter record low in 2017 it is looking unlikely that the summer will also set a record for extent, but there is still a month to go.
We understand that sea ice melt is erratic – we should not expect new records every year, but the overall trend is towards less and less ice. But, what about looking further ahead? And, can we understand sea ice variations during the historical period? Continue reading Linking global temperature and Arctic sea ice changes