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The targets set in the proposed Green New Deal are a bit ambiguous. It suggests a 10-year mobilization, but does not necessarily set a goal of net-zero carbon by 2030. Lets explore the impact of the goals on the climate and the challenge of mitigation *epic thread*. 1/27
First, the climate. If the world wants to limit warming to below 1.5C (with a ~66% chance of avoiding it) with limited deployment of negative emissions, US emissions would have to fall around 50% by 2030 and to net-zero a bit after 2040. 2/
Equity considerations (giving more room for future emissions from poor countries) could further compress this timeline, as would divvying up the remaining carbon budget evenly to each country based on population. 3/
If we allow large-scale negative emissions later in the century, integrated assessment models have the rich OECD countries (including the US) reaching net-zero emissions between 2050 and 2060 in a scenario. 4/
If we aim for a 2C target instead, it buys us a bit more time, with emissions needing to reach zero by around 2070. Folks can play around with the model runs in the IPCC SR15 database here: data.ene.iiasa.ac.at/iamc-1.5c-expl… 5/
While there are some pretty big differences between 1.5C and 2C warming, especially for fragile ecosystems like coral reefs and small island nations, our best estimate is that limiting warming to 2C wouldn't impose great hardship on most of humanity. interactive.carbonbrief.org/impacts-climat… 6/
So what if we actually wanted to reduce emissions to zero in the US in 10 years, or in 30 years? What would that look like? Lets go sector by sector and see what it would entail. 7/ 
Lets start with power generation, which in many ways would be the easiest to deal with as the emissions are the most centralized and replacement zero-carbon alternatives are the most technologically mature. 8/
Completely decarbonizing the power sector in 10 years would be a big undertaking. The majority of existing plants would have to be retired well before the end of their useful life, including a lot of natural gas capacity built in recent years. 9/
Scaling up alternative energy that quickly would likely leave little time for environmental reviews, and would probably require permitting just about every zero-carbon project proposed as @alannogee points out 10/
It would also require tackling the intermittency issue in a big way, as there would be no natural gas backup for renewables. This would require huge investments in a high-tech power grid to transfer massive amounts of power long distances and large-scale battery storage. 11/
By contrast, a 30-year decarbonization target would let most plants retire at the end of their life – though we'd still want to phase out coal as quickly as possible to maximize the utility of our remaining emissions budget. 12/
Natural gas could help support the large-scale growth of renewables, slowly ramping down as batteries become cheaper, transmission options improve, or other non-intermittent zero-carbon electricity like nuclear play a larger role (assuming new plant costs fall by then). 13/
Now, the power sector is the easy one. Commercial and residential, for example, will be considerably harder. There emissions are primarily from home and building space and water heating. 14/
Most building systems are expected to be in place for more than a decade, so decarbonizing by 2030 would require buying up every gas, oil, or propane space or water heater in the US, replacing them with electric versions. Biogas could help, but would be tough at scale by 2030 15/
A 30-year decarbonization target would allow more wiggle room for the end-of-life retirement of existing systems, though it would still mean that new construction would start having to use electric systems immediately at large scale. 16/
Transportation suffers from similar problems, with a large portion of the internal combustion engine vehicles in-use today expected to remain so by 2030. Decarbonizing transportation in 10 years requires buying up a large portion of US vehicles. 17/
Similarly, it unlikely that large-scale zero-carbon aviation alternatives will be available within the next decade, leaving it unclear how air travel would continue under a 10-year decarbonization target. 18/
Even decarbonizing transportation in 30 years would prove to be a challenge, as new cars would have to start being solely electric (or other alternative fuel) rather quickly, and its unclear how long it will take to decarbonize aviation. 19/
Industry emits CO2 greenhouse gases through a variety of processes, but the bulk comes from burning fuel to produce heat. Unlike in the case of electricity generation, where most of the heat is lost in the conversion to electricity, these processes tend to be more efficient. 20/
This means that replacing fossil fuels with electricity-to-heat for industrial processes would likely be much costlier than replacing fossil fuels for electricity generation. Other options like biofuels could help, but 10 years is short timeframe for deployment at scale. 21/
A 30-year decarbonization pathway for industry would allow the end-of-life retirement of some fossil fuel equipment, the deployment of more electric alternatives, and more development of biofuel-based options for both new and existing equipment. 22/
Finally, non-CO2 greenhouse gases will pose a challenge in both timeframes. Some methane emissions can be mitigated through leak remediation and reduced oil and gas production. However, many agricultural emissions will be harder to reduce. 23/
Reducing agricultural emissions to zero in 10 years would require forced dietary transitions away from red meat and dairy, as well as the slaughter of most existing herds of ruminants. 24/
A 30 year period would allow more development of meat and dairy alternatives (e.g. lab-grown meat) that do not involve incidental methane emissions, though some methane emissions from agriculture will remain as long as we have cows. 25/
Overall, it is much easier to see a realistic pathway to decarbonize in 30 years rather than 10, and doing so does not preclude meeting Paris Agreement targets of limiting warming well-below 2C. Even 1.5C is possible with large-scale negative emissions. 26/
That said, if action stagnates for the next 20 years, a 10-year decarbonization would be nearly as hard to achieve at that point as it would today. A longer decarbonization timeframe does not obviate the need for mitigation efforts to begin today. FIN
