In the late 1990s, Greenland and Antarctica were thought of as enormous but slow-changing freshwater reservoirs. We knew their waxing/waning during glacial cycles had caused sea level to rise/fall by 400 ft, but this had happened over 10s of 1000s of years.

A long icy🧵. 1/24
A big wake-up call came in the early 2000s, when Antarctica’s Larsen B Ice Shelf collapsed in 2002 in a matter of weeks, and huge outlet glaciers in southeast and west Greenland, such as Jakobshavn, Helheim and Kangerlussuaq glaciers lost their floating tongues. 2/24
It was not so much the collapse of floating ice that caused alarm – it’s already floating – but the ice behind the ice shelves, that sits on land, began to flow faster into the ocean. 3/24
That's because ice shelves act like a buttress for the grounded ice. If that buttress weakens, the ice it is holding back will flow faster. 4/24
Satellite data showed us that similar changes were occurring all around West Antarctica (Amundsen and Bellingshausen seas) and Greenland. Floating ice was thinning and/or breaking off and the ice behind it was moving faster into the ocean. 5/24
The scientific community got to work to understand what was happening, and came to a consensus that these changes were due to increased melting above the waterline (warming atmosphere), and increased melting below the waterline (warming ocean). 6/24
Warming of oceans reaching the ice sheets was not surprising – we knew the ocean had absorbed about 90% of the extra heat accumulated due to increased greenhouse gases. We also knew air temperatures were rising globally, especially in the Arctic. 7/24
What was surprising was the speed: the ice sheets could no longer be thought of as sleeping giants that took centuries to respond to climate change. But this had not been predicted by models. 8/24
In the 1990s, the @IPCC_CH's 1st and 2nd assessment reports concluded that ice sheets changed too slowly for us to worry about them. These are the reports many of us who are now “mid-career” had on our desks when we were PhD students just entering the field. 9/24
In 2007, in response to the rapid changes of the 2000s, @IPCC_CH challenged the polar science community in its 4th Assessment Report to up the world’s understanding of ice sheets. They concluded that we didn’t know enough about ice sheet changes to include in models. 10/24
This motivated the ice community to change its thinking about ice sheets and realign research priorities, launching several campaigns to combine computer simulations of ice flow with new data coming in from satellites and from teams working on the ice and the ocean. 11/24
In 2014, the 5th IPCC report contained some initial estimates of future mass loss, which included dynamic changes, based on four carbon emission scenarios from best-case to worst-case. The Paris Agreement came that same year. 12/24
In 2018, things were changing so quickly that the update couldn’t wait, prompting an intermediate report -- the Special Report on the Ocean and Cryosphere in a Changing Climate. The projected sea level rise in 2100 from Antarctica and Greenland ranged from 3.5" to 1.8'. 13/24
In 2022, the upcoming IPCC report will contain updated projections of future sea level rise based on tens of different simulations provided by research groups around the world. 14/24
These groups all worked together in a community-led effort, involving ice sheet, ocean and atmosphere modeling and observational teams, funded by many sources including @NASA, @NSF & @NERCPlanetEarth. 15/24
We've come a long way, but even after all this we're still playing “catch-up”, and there is still a lot to learn. We do know, however, that the ocean is warming and that both Antarctica and Greenland are vulnerable to this warming. Same for the atmosphere. 16/24
We worry that the biggest portion of Antarctica, East Antarctica, which is the sleepiest giant since it is so thick and vast, making it harder for warming ocean waters and increasing air temperatures to reach it, is starting to signs of change. 17/24
We also worry that there may be mechanisms, that we have not been able to witness in the modern record and hence that are not in the models, that may amplify the ice loss. Scientists are using paleo-reconstructions to figure out whether these may be important. 18/24
We do know that sea level will continue to rise (faster) in the future, and that our projections are conservative estimates. We know this because satellite observations of ice sheets are tracking the worst-case predictions. 19/24
As we gather more data, both on and around the ice sheets using all available tools, including satellites, our observational record gets longer and our understanding improves. As our understanding improves, our models get better. 20/24
Long-term measurements, sometimes acquired by launching new satellites (such as @NASA @NASA_ICE ICESat-2 and other follow-on missions), coordinated modeling and international collaboration with @esa @esa_cryosat are key to delivering more accurate predictions. 21/24
This is vital so that coastal communities around the globe can make informed decisions to protect infrastructure and citizens and manage resources as sea level rises. 22/24
There are so many incredible people involved in all of this work, across both ice sheets, and all around the world, too many to mention individually. This is a community effort, and that is the point: large, collaborative, inclusive teams get important work done! 24/24/end

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More from @helenafricker

7 Nov 20
This is what this means to me: as most of you know, I am British, and always will be. But, in February 2017, we became US citizens, after nearly 18 years here. For me it was bittersweet, I just could not get into it, it all felt wrong; I felt I was selling part of my soul. 1/6
When we came out after the ceremony, there was a cardboard cut-out of President Obama and Michelle Obama by the Democratic party booth, and I had my photo taken next to them as I registered to vote. 2/6 Image
For the next 3.9 years I watched this country get suffocated by a president with no interest in the common good: dividing us all; inciting hate against anyone white, straight and able; totally disregarding science. Literally he assaulted every single one of my values. 3/6
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