Nature: Our Best Climate Technology?
Personally, I am convinced that we can solve the whole transportation emissions question overnight by switching from fossil fuel to biofuel - by adding charcoal gasification to ICE cars so they can run on regeneratively-grown charcoal pellets derived from pelletized biomass. Charcoal pellets are a flowable fuel, and requires minimum car modification such that new cars could ship with this option. Agriculture wise, this can be integrated with perennial polyculture plantings, such as those including hazelnuts and chestnuts (see Badgersett Research) that can be coppiced to produce fuel in addition to food.
We are developing this option with Open Source Ecology (please see my TED Talk for context, https://www.ted.com/talks/marcin_jakubowski), based on first principle calculations. Take the case of 380 million gallon Fuel Use in the USA. Take the area covered by roadways in the USA.
By James Bowkett:
Historically civilisation was built on biomass, it failed to provide sufficient fuel to allow development and resulted in multiple collapsed civilisations. A city without resources for building & fuel will collapse, as such the use of biomass puts strict limits on the possible scale of towns/cities. So long as the size of a community is strictly limited according to the resources available biomass may be successful.
Forests are a significant Carbon Sink, is this the case for biomass crops? What is the mass of a coppiced woodland in comparison to a mature forest with large-scale lumber? Coppicing deprives the community of dimensional lumber for construction. While we might believe we will only take what is needed it is probable that at some stage less scrupulous individuals will gain control of resources and over-exploit them.
Pollutants, and calculation of pollutants released by biomass; It's been suggested that NOx is the chief pollutant gas from biomass burning, certainly the pollutants from biomass are less from say coal but I struggle with the idea that NOx is the only pollutant.
Of course, all the challenges above can be addressed; diverse energy sources, clear fair policy & regulation, etc... will go a long way to alleviate problems.
Recommended Reading: 
Case 1: Taking out all roads and planting them with perennial polyculture biomass
- Acreage of Highways - 24 acres per mile, or about 1M acres
- Length of USA Roads - over 4 million miles, with 48,000 miles of USA Interstate Highways. If average road is 30 feet wide  - that area is
- 3 acres per mile - so about 14M acres of non-highway roads.
- Harvest from area of all roads: 15Mx1000 lb = 15B lb.Sustainable harvest biomass yields are 1000 lb per acre per year. This assumes integrated agroecology of perennial agriculture.
- Top biomass crops with such as switchgrass are 5 tons/acre , and oil palm is much more.
- For sustainable harvest biomass, get 200 lb charcoal/acre, or 3B lb/year - assuming 20% efficiency of charcoal production from dry biomass weight (ie, 5 lb of dry wood turn to 1 lb of charcoal)
- Thus, if total USA fuel use is 380 million gallons, or about 2.5B lb - and suply of charcoal is 3B lb for sustainably harvested biomass - then just covering the area of US roads with biomass production yields half the total fuel supply of the USA.
- assumes energy per mass of charcoal is equivalent for charcoal and gasoline [ref?], and 1 gallon of gasoline weighs about 7 lb [ref]
-  wikipedia article states that current biomass energy is 1.4x larger than all human energy consumption.
- Charcoal Yields
Case 2: Using Farmland
- There are 922 million acres of farmland in the USA, see Stats, and 922B lb of regeneratively-harvested perennial polyculture biomass crop - and 1/5 of that for charcoal yield - 180B lb - or 90B lb fuel equivalent.
- Need in the USA is 2.5B lb per year.
- Therefore - regeneratively-grown biomass can supply 30x all the transportation fuel in the USA. Or - 3% of farmland would have to be augmented to perennial polyculture to yield all the fuel use in the USA. Because it can be integrated with other use, food production can in principle increase if for example coppiced food/fuel crops are used. Compared to batteries, this is regenerative. Batteries aren't.
Road percentage compared to land area: 1%. And 2% if farmland is considered.
David Mackay calculated that If all the cars on a road were powered using biofuels grown on the verge the planted verge would need to be 8km wide. Is this calculation more pessimistic because it is more efficient to use charcoal in gasification compared with converting biomass to make liquid biofuels?
Richard Patton estimates that it is possible to replace 75% of current diesel use in the USA with sustainably grown charcoal in a charcoal-water slurry fuel for diesel engines.
In a Biomass-to-Wheel efficiency analysis, Huang and Zhang estimate that in the longer term, 7% of annual US biomass production could fuel 100% of light-duty passenger vehicle fuel needs with Sugar-Fuel-Cell technology.