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Special Climate Central Report on Best Cars for Climate

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Methodology

Our calculations begin with the EPA’s combined highway/city driving fuel economy of cars: miles per gallon for gasoline cars and kilowatt-hours per mile for electric cars. (See Appendix Table A1 of Full Report)

For a gasoline car, the bulk of the lifecycle greenhouse gas (GHG) emissions associated with driving are due to the CO2 emitted by combustion of the fuel in the car’s engine. A gallon of gasoline releases about 19 lbs of CO2 when burned, or about three times the weight of the gallon before it burns. To these CO2 emissions we must add the GHG emissions associated with extracting, transporting, and refining the crude oil used to make that gallon of gasoline. When these are included, the total lifecycle GHG emissions for using gasoline in a car come to 25.9 lbs of CO2-equivalent per gallon. (a)

In this report, the term CO2-equivalent (or CO2e) is used to refer to GHG emissions. This measure expresses the combined global warming impact of several different gases in terms of the amount of CO2 alone that would give the same warming. (GHGs in addition to CO2, such as methane (CH4) and nitrous oxide (N2O) are emitted over the lifecycles considered here.) Since different gases have different lifetimes in the atmosphere, the relative warming impact of the non-CO2 molecules depends on the time frame under consideration. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (b) gives global warming potentials (GWPs) relative to CO2 for a large number of gases considering 20-year, 100-year, and 500-year time frames. For the results reported in the main body of this report, we have used the 20-year GWP values since 20 years is close to the typical lifetime of a car — certainly much closer than either 100 years or 500 years. Our results recalculated using 100-year GWP values are given for comparison in the Appendix (Tables A2 and Table A3 of Full Report). (For a gasoline car, the lifecycle emissions assuming a 100-year GWP are 24.3 lbs CO2e/gallon instead of 25.9 for a 20-year GWP(a))

Estimating GHG emissions associated with electricity use by an electric car is more difficult than estimating emissions for a gasoline car, because it is essentially impossible to say with certainty that an electron generated at a particular power plant is the same electron that ends up in the battery of a particular vehicle. The uncertainties arise because of the nature of electricity flow and the geographical extent and interconnectedness of electricity grids. (c) Additional uncertainty is introduced by the time-varying nature of electricity demand and supply. For example, if an electric vehicle plugs in to charge during a period of peak electricity demand, the mix of power plants generating electricity (and hence the GHG emission profile of the electricity) will be different from the mix of plants during periods of lower electricity use. In general, the greater the temporal or geographic specificity with which we wish to determine the emissions associated with electricity use, the greater will be the uncertainty around whether the emissions accurately represent actual use.

To make our analysis tractable, we have chosen not to consider time-of-use variations in electricity emissions, choosing instead to use annual emissions per megawatt-hour generated from power plants. We also assume electricity generated in a state is consumed in that state. A recent similar study by the Union of Concerned Scientists (d) also uses annual emissions per megawatt hour, but chooses to divide the U.S. into 26 electricity-generating/consuming sub-regions defined by the EPA. (e) Another study in 2007 by the Electric Power Research Institute and the Natural Resources Defense Council divided the U.S. into 13 sub-regions. (f)

The larger the geographic region selected, the more certain one can be of the average emissions associated with each kilowatt-hour used in that region — for example, the average emissions per kilowatt-hour consumed for the entire U.S. can be known with considerable certainty. The drawback of averaging over larger and larger areas is that less and less insight can be gained into the impact of geographic distribution of different electricity generating sources. In an effort to balance these competing considerations, we have chosen to average emissions at the state level. For large states, or for states of any size that have similar electricity generating fuel mixes as neighboring states, the uncertainty introduced by this assumption is small. The uncertainties are larger for smaller states.

To estimate state-level GHG emissions associated with electricity, the following methodology was adopted. The starting point were data published by the Energy Information Administration (EIA) on how much CO2 was emitted on average per kilowatt-2 hour (kWh) of electricity generated in each state in 2010 (TableA4 and Figure A1 of Full Report). (g) This average is most influenced by the types of fuels used in the power plants in the state. For example, a state that relies more on nuclear or hydro power will have lower average CO2 emissions per kWh generated than a natural gas-reliant state or, especially, a coal-reliant state. But CO2 emissions at a power plant alone are not the full emissions story because there are also emissions associated with supplying fuel to the plant (e.g., emissions that occur during coal mining or natural gas extraction). Accurately estimating on a state-by-state basis the emissions other than those at the power plant itself requires detailed lifecycle calculations.

These calculations were undertaken using the Argonne National Laboratory’s Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model, version 1_2011, the same model used to estimate the lifecycle GHG emissions for gasoline mentioned above. GREET was run for each state’s electricity system by specifying in GREET the mix of fuels used for electricity generation in the state. (g) The main sources of electricity in any given state in the U.S. are coal, natural gas, nuclear, and/or hydro. (Renewables other than hydro play small roles in most states today.) In the case of natural gas, the power plant technologies used vary significantly from state to state, and the average efficiency of generation from natural gas varies accordingly from state to state. (This is not the case for coal, nuclear, or hydro plants.) Efficiency directly impacts the GHG emissions per unit of electricity produced, so we provided the mix of natural gas power plant technologies in each state as an input to GREET. The mix of natural gas powerplant technologies (combined cycle, simple cycle, or steam cycle) in each state was obtained from EIA data. (h) GREET’s default values for electricity generating efficiencies were then kept for all power plant technologies.

The outputs from running the GREET model for each state include A, the average CO2 emissions at power plants per kWh generated and B, the average total lifecycle GHG emissions in CO2- equivalents per kWh delivered to the end user. (The transmission and distribution losses assumed by the GREET model are 8% of generated electricity.) For each state, the ratio B/A was calculated and multiplied by the average CO2 emissions per kWh of electricity generated (derived from EIA data, as described above) to arrive at the lifecycle GHG emissions associated with electricity used in each state in 2010. For each state, separate calculations were done using 20-year and 100-yr GWP values for non-CO2 gases (Table A4 of Full Report).


(a) This estimate is for gasoline from conventional crude oil, as calculated by the Argonne National Laboratory’s Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model, version 1_2011. (See Figure 2 in A. Burnham, J. Han, C.E. Clark, M. Wang, J.B. Dunn, and I. Palou-Rivera, “Life-Cycle Greenhouse Gas Emissions of Shale Gas, Natural Gas, Coal, and Petroleum,” Environmental Science & Technology, 46: 619-627, 2012.)

(b) IPCC, 2007: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Available at www.ipcc.ch.

(c) For a thoughtful discussion of this issue, see C.L. Weber, P. Jaramillo, J. Marriott, and C. Samaras, “Life Cycle Assessment and Grid Electricity: What Do We Know and What Can We Know,” Environmental Science & Technology, 44: 1895-1901, 2010.

(d) Anair, D. and Mahmassani, A., “State of Charge: Electric Vehicles’ Global Warming Emissions and Fuel-Cost Savings Across the United States,” Union of Concerned Scientists, (prepublication version), April 2012.

(e) The UCS report uses an overall methodology quite similar to the one we have used. Two notable differences are in some key input assumptions, including the use of 2007 emissions data by the UCS (rather than the more recent 2010 data we have used) and 100-yr GWP values for estimating the global warming impact of non-CO2 greenhouse gases. We have used a 20-yr GWP, but also show results for 100-yr GWP in the Appendix.

(f) EPRI and NRDC, “Environmental Assessment of Plug-In Hybrid Electric Vehicles, Vol. 1: Nationwide Greenhouse Gas Emissions,” July 2007.

(g) Energy Information Agency, State Electricity Profiles 2010, US Department of Energy, January 2012. (In this reference, Table 5 for each state gives annual electricity generation by fuel type, and Table 7 gives CO2 emissions from electricity generators.)

(h) Theamountsofelectricitygeneratedineachstatein2010brokendownbytypeofpowerplanttechnologyarecollectedbythe Energy Information Administration on form EIA-923 and published in spreadsheet form. For input to GREET, the natural gas generating technologies were categorized as combined cycle, simple cycle, or steam cycle.

Page 1: Report Summary
Page 2: Press Release
Page 3: Methodology

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Comments

By Eric
on April 26th, 2012

Kevin: I love it! a diesel Beetle!  If you get 50 mpg on diesel, that’s about 44 mpg gasoline equivalent due to the difference in energy density of gasoline and diesel. (We showed all results in our study in terms of mpg of gasoline equivalent.) Figure 6 in our report says that with Tennessee’s electricity generating mix you would need to get at least 47 mpg-ge to be a lower carbon emitter than an all-electric Leaf. Of the cars that we evaluated, only a Prius (at 50 mpg on gasoline) would be able to do that. But if you are really gettting 44 mpg-ge, I would say you aren’t far from the best you can be doing—your emissions are certainly far lower than for the average car on the road today.

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By Eric
on April 26th, 2012

Bethina - I like your call for a paradigm shift!  But I would respectfully disagree with your point about the impact that car manufacturing has on lifecycle GHG emissions. You have correctly pointed out that we did not include this in our calculations, but the reason we left it out is because it is actually a pretty small number. The study on this topic that I am most familiar with is one published in 2007 by Kromer and Heywood from the MIT automotive lab. They found that GHG emissions from manufacturing a hybrid (like a Prius) addes about 1% to the vehicle’s lifecycle emissions. For a plug-in hybrid car they found a 2% increase.  They didn’t do the calculations for an all-electric car.  If you know of reputable studies that have found a large impact on GHG emissions from manufacturing of automobiles, I would love to know about them.

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By Eric
on April 26th, 2012

James - You have a good understanding of how our electricity system operates! We mention in our report that we chose to use each state’s annual average CO2 footprint, which may be different from the footprint of marginal generation at any given time, as you have suggested. We decided to go this route because there are too many uncertainties both on the supply side and the demand side to try to use a marginal analysis. For example, if the electric car is charged during a period of low overall system electricity demand, the marginal kWh will come from one type of power generator. If it charges during a time of system peak demand, the marginal kWh will probably come from a completely different source. We would have needed to make assumptions about when people would charge their electric cars and correlate these with time-resolved (at least hour-by-hour) power-plant operating data. I’d love to see someone take on this project to see how different the results are from ours and how sensitive to assumptions made about when people charge their cars.

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By Nate (Seattle, WA, 98116)
on April 26th, 2012

James and Eric,
Attempting to perform accounting for electric cars solely based on marginal electricity consumption would be completely bogus, for at least a couple reasons.

1) If a family uses 1000 kWh with an electric car, and 500 kWh without it, who’s to say which 500 kWh should be counted at the lower utility rates, or counted as drawing from the more environmentally friendly energy source?  The 500 kWh that you were using “first”?  That’s completely arbitrary and ridiculous.  If 500 kWh are used to transport you to and from your job, but 100 kWh are used to power your hot tub or home sauna, do you consider the car, which is used for a more essential purpose, to be using “marginal” electricity, and the luxury spa, sauna, big screen TV, excessive overhead can lighting, etc. to be using “average” electricity? Or even worse, using the most environmentally friendly source? Sorry, but that doesn’t pass logical muster.

2) The same exact effect exists for gas engine cars.  When we had lower demand for oil, we found oil that was easily extracted by pumping in the US, or more recently, in Saudi Arabia.  That oil could be extracted with relatively low amounts of energy being expended to get the oil.  As we’ve used up that oil, now we have to drill deeper and deeper offshore, and extract oil from tar sands, which are much more energy intensive and expensive operations.  And remember, an electric car is actually reducing how much oil is used.  So, if you’re going to do accounting for electric cars based on marginal factors, you certainly have to do the same for oil.

The fundamental problem is that because of the way electricity and oil are priced in this country, people are conditioned to think about the marginal cost of electricity being higher than average, but they don’t realize that the same issue exists with oil (they don’t notice this because oil is priced to let you use all you want for the same price).

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By Barbara (Texas)
on April 27th, 2012

In Texas, we have a fair amount of wind power on the grid, and through the power to choose program, we can support wind by choosing an “all wind” electricity plan. Now I understand that doesn’t mean only wind-powered electricity comes to my house per se, but that it’s an equivalency situation. Still, it seems like your analysis should make some allowances for this growing inclusion of wind in the ERCOT…and this scenario: when the grid is using the most wind, at night, it’s also likely to be recharging electric cars in home garages….I’d love to hear your thinking on this.

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By Dallas May (Dallas)
on April 27th, 2012

This report seems unlikely. First of all, when you plug in your Leaf, coal plants don’t start producing more CO2 on your behalf. They are always producing the same amount of CO2 regardless of whether or not you have your leaf plugged in. Now, they do burn more at different times of the day, and different days of the year, but they are on a schedule that doesn’t depend on your Leaf being plugged in.

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By bobby (02134)
on April 27th, 2012

Power plant efficiency varies greatly depending on system demand and type of plant. A fossil fuel power plant must burn a steady supply of fuel at all times during peak months. This is because of the daily demand cycle, low at night high during the day. You can’t easily throttle back and forth a coal/oil/nuclear plant. What is done instead is excess steam is vented and generators are taken offline at night. All the while the same amount of fuel is used. This is done because the plant needs to be able to respond quickly to a jump in demand. Therefore plant efficiency is greater during the day.

Most electric cars will be charged at night. This will allow plant operators to keep their generators online. Plants will remain at a higher efficiency. However CO2 emissions remain the same for the plant day or night.

Natural Gas turbines are used because they can be throttled up and down easily. Their efficiency remains the same, but their CO2 emissions change according to demand. When possible power companies supplement coal/nuclear plants with natural turbines which can act as boosters to the grid. This allows the traditional steam plants to operate at full capacity at all times (max efficiency), and the natural gas generators kick in during peak demand hours. However this is not the case in many areas’s which still operate by simply venting excess steam at night.

Supply side things do not change as you suggest. The core of the power industry is still steam. The trend is small gas fired turbines that soak up demand as needed.

By charging an electric car at night, we are for the most part, not increasing overall power plant CO2 emissions.
On the other side of things we also look at the oil refining process. The CO2 generated via transporting the gasoline from the refinery to the pump is not taken into consideration. Also, oil is refined via fractional distillation. Lighter fuels on top and heaver ones on the bottom. Only 40% of one barrel of oil can be made into gasoline. In order to get gasoline we are forced to deal with the leftovers.That is a huge loss. If we were not refining to produce gasoline we would end up with much heaver byproducts. More heating oil/diesel. We are essentially refining oil to meet gasoline demand.

So for every gasoline powered vehicle that we take off the road we reduce the demand for gasoline (no brainer). This allows us to refine for the demand for heaver fuels. Which we get more of per barrel of crude. Making the refining process much more efficient because we can refine less crude to meet our energy needs. This reduces the overall CO2 cycle associated with getting crude from the ground to the refinery, and eliminates the CO2 generated from the refinery to the pump.

There are many misconceptions about the power industry in general. That is simply because we fail to grasp how electricity is generated and the overall big picture. I have read many reports from high level people who fail to understand simply because they have never stepped foot in a power plant. That is just how it is. Researcher’s research and engineers engineer.

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By John
on April 28th, 2012

This is ridiculous propoganda.  Shame on you.  Climate central…. My ***.  Another petro front group and we all know it.

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By Eric Larson
on April 30th, 2012

John: Thanks for your comment. You seem to have some confusion about Climate Central. We are not a “petro front group”. You can see who funds the organization here: http://www.climatecentral.org/what-we-do/funding/

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By HH
on May 1st, 2012

Interesting, but the ‘conclusions’ may be flawed. Have a look at Bobby’s comment above—nighttime recharging is VERY different from daytime.

The ‘conclusions’ reached by news reports of this are definitely flawed: there are plenty of good reasons to buy an electric car now:
1. The CO2e savings may not be as big as we’d like, but we also know that in the short term black carbon (particulate matter) is also contributing to the problem. In the long term, electricity will be cleaner and the transformation of the transportation system to electric requires buy-in and slow ramp-up. A future with renewable electricity but dirty vehicles doesn’t get us to where we need to be in the coming decades, and although there is a place for biofuel on the roads at least as an intermediate, that can’t be the large-scale solution for cars—those need to be reserved for aviation and possibly heavy vehicles.

2. Air quality benefits are huge, and in the right place. Vehicles impact health much more directly because they are the nearest source to people.

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By JH (Austin, Tx 78746)
on May 2nd, 2012

The conclusions seem deeply flawed.  In some states buying renewable electricity does not work, in others it does.  If the purchaser of the leaf or volt buys green choice how many states will the carbon footprint be lower.  What will it be over the lifetime of the car as it ages.  The thrust of buying renewables creates demand for more renewables and a faster cleaning of the grid.  This is especially true in my state of texas where night charging will bring more wind on line. 

How many of these plug-ins go to the dirty states versus the clean.  IIRC 60% of leaf sales went to california?

Why use 50% for volt gasoline usage.  voltstats and onstar peg the real number at 60%-70%. 

How many leaf, tesla, volt drivers would be satisfied with a prius.  The headline will make them go buy a less efficient, higher ghg model instead of looking for a plug in.  The prius phv and ford energis also are hurt by the misleading headlines associated to this report.

IMHO as oil gets more scarce, if there is not a more now to electricity, there will be no money to clean up the grid.  Misleading reports like this make it sound like plug-ins are part of the global warming problem, not the solution.  This only compounds the problems.

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By Frank Niepold (Brookeville, MD 20833)
on May 4th, 2012

The conclusions miss some key points.  I live in Maryland and am able to purchase 100% wind produced electricity from Maryland wind farms at a lower rate than coal.  This is done through the Renewable Energy Certificates (RECs), http://www.epa.gov/greenpower/gpmarket/rec.htm.    Here is how our community became a Green Power Community, http://www.epa.gov/greenpower/communities/communities/brookevillemdcommunity.htm.  If I could finance an electric car, I would absolutely go electric.

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By Christopher Miles (Philadelphia, Pa 19129)
on May 11th, 2012

Thanks for this starter chart

Perhaps a bit more on the high mileage Domestics (like the 2012 Chevy Cruze) My real world highway mileage with the 1.4l turbo is 40+ MPG, and I don’t have the ECO model.

Cruze is a good baseline as it’s the same platform as the Volt; and it’s 12K cheaper.

What about an additional table/row/cell for N0x and HC?

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By SecularAnimist
on May 25th, 2012

It appears that this analysis does not take into account the ability of electricity consumers in some states to choose their supplier.  Like a previous commenter, I live in Maryland where your chart indicates that 54 percent of generation capacity is coal-fired.  However, I purchase 100 percent wind-generated electricity through the Clean Currents Neighborhood Wind program (the electricity producer is the Big Sky Wind Farm in Illinois).  So in my case, powering a fully electric car like the Leaf would have, if not a zero carbon footprint, than certainly a far smaller footprint than your chart would suggest.

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By Todd (La Crosse/WI/54603)
on May 25th, 2012

Flawed!
1. Electric cars and more than twice as efficient as ICE cars (30% max).
2. Most coal plants are more than twice as efficient ICEs
3. If you have some of your own solar/wind/? clean power, it won’t help a gas car.
Even if you have coal electric, it’s still less greenhouse than ICE engines, and that’s what’s pushing us closer to the POINT OF NO RETURN!!

ELECTRIC IS THE FUTURE ALL THE WAY!

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By Bob (Burbank/CA/91506)
on October 5th, 2012

Some points:

1) Seems you’re comparing apples to oranges by ignoring weight: the Leaf seats five and weighs 314 lbs more than a Prius, and 500 lbs more than a Honda Civic Hybrid. It’s a bigger car.
2) Why did you choose to ignore particulates in your assessment of “greenhouse gas pollution” (or actually, that only gases should be considered in determining climate-friendliness)?
3) State by state? For West Virginia, for example, a more accurate picture would be to spread out generation over the 11-state synchronous grid which provides power for WV, creating a multi-state average with far lower utility emissions than WV alone.
4) The car you buy today won’t be the same car you drive in 2017: a gasoline-powered car’s emissions very reliably increase over time.
5) Electrical generation very reliably gets cleaner over time (with cleaner fuels and installation of electrostatic scrubbers).

With these points in mind it seems obvious that continued reliance on fossil fuels is anything but forward-looking.

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By Tom Willis
on January 5th, 2013

The report and article keep saying “fewer emissions” when they mean “less emissions”. Saying “fewer emissions” implies the *number* of things being emitted is smaller; saying “less emissions” means the *amount* of stuff being emitted is smaller.
Please could this be corrected?

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By Barbara Zaveruha (Northfield, MN 55057)
on February 5th, 2013

Minnesota is tagged as having high-carbon electricity.  But our utility co-op has an option to sign up and buy wind power instead of coal power.  I can buy all wind-generated electricity for a nominal surcharge ($4 to $10 per month).  At which point, a plug-in electric car is not contributing any fossil CO2 to the atmosphere.

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By SlowMoneyGreen (Brentwood, CA 94513)
on February 21st, 2013

We’ve had a Chevy Volt for about 3 months now.  In the last 1500ish miles, it’s used 4.2 gallons of gas and about $50/mo of electricity.  We’re fortunate where we live, in that our only power sources are a brand new Nat Gas plant and the huge local wind resources. 

It get where you’re going with all this (the power source matters, as does the supply chain, and any dirty energy inputs), but we’re not there yet…not the nation.  We’re dipping our toes into these cars.  These aren’t the cars that will save the planet, but rather these are the cars that will get people talking, and start the infrastructure rolling. 

If we all plugged into tomorrow, then we’d be screwed.  The grid needs to smarten up, to tap these cars as our common energy storage system.  The nation needs time to build the electric generation capacity, and if We The Greens push the cars to hard, we’ll end up with coal powered electric cars.  Gah!

Be patient, and move to build up the infrastructure, as we with these cars tear up the roads showing off their amazing performance.  Did I mention I love this Volt?  Amazing vehicle….

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By Tom Grizzle (Chapel Hill, NC 27517)
on May 6th, 2013

The car that will “save the planet” is the one that has been recycled into something non-car. At present, I maintain that the “greenest” car (I hate that term too!) is the one that does not have to be manufactured. We have something like 300M cars in the USA. Isn’t that enough? What if we just fixed what we already have and stopped there? No more mining for metals and oil for plastics and rare earth elements for the various components in cars. How much emissions does all this activity create?  VAST! 

Hybrids have two, count them, two motors, and some with a huge battery with relatively rare metals.  Green my gluteus maximus!  Repeat, the greenest hybrid is the one that does not get manufactured in the first place, period!

I am still forming my opinions regarding electric cars. They have to be manufactured too. There’s practically no discussion of the energy embodied in these car-shaped objects.  And just because you charge it from a solar panel, they too have to be manufactured and some have rare earth elements.  How much emissions does all this activity create?  Again, VAST! 

Folks, it’s time to face reality. Cars have to go.

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