3032. Terrestrial Primary Production: Fuel for Life

By Knowledge Project, The Nature Education, No Date

Terrestrial ecosystems rely almost exclusively on the sun's energy to support the growth and metabolism of their resident organisms. Plants are quite literally biomass factories powered by sunlight, supplying organisms higher up the food chain with energy and the structural building blocks of life. Land plants, or autotrophs, are terrestrial primary producers: organisms that manufacture, through photosynthesis, new organic molecules such as carbohydrates and lipids from raw inorganic materials (CO₂, water, mineral nutrients). These newly minted organic compounds lock up the sun's energy in chemical bonds, providing an energy currency accessible to heterotrophs, organisms that consume rather than produce organic molecules. In this way, primary producers are an essential vehicle for energy transfer from the sun to consumers, securing energy that can be passed from one consumer to another. The energetic and carbon-rich products of primary production supply consumers, including humans, with fuel to drive their metabolism while providing essential carbon-containing compounds that form the bricks and mortar of living cells.

Ecosystem ecologists have long been interested in two related metrics of terrestrial primary production. Gross primary production (GPP) is the total amount of carbon dioxide "fixed" by land plants per unit time through the photosynthetic reduction of CO₂ into organic compounds. A substantial fraction of GPP supports plant autotrophic respiration (R_a), with the remainder allocated to the net primary production (NPP) of plant structural biomass in stems, leaves, and fruit, labile carbohydrates such as sugars and starch, and, to a much lesser extent, volatile organic compounds used in plant defense and signaling. Terrestrial GPP, therefore, relates to NPP as follows:

NPP = GPP - R_a

Figure 1: Net primary production (NPP) and standing biomass allocation for a 90-year-old Michigan forest estimated from inventory-based methods in which biomass growth is quantified over time (Gough et al.2008)

© 2011 Nature Education All rights reserved. 

Both GPP and NPP are expressed as rates, usually in terms of their carbon currency (e.g., g C m^-2 hr^-1, tonnes C ha^-1 yr^-1). Because volatile organic compounds represent only a small fraction of NPP, the rate of total plant growth (or yield) in a terrestrial ecosystem is virtually synonymous with NPP, since biomass production is already discounted for respiratory expenditures that support plant growth and maintenance. The ratio of NPP to GPP, or carbon use efficiency, is the fraction of carbon absorbed by an ecosystem that is allocated to plant biomass production. Interestingly, carbon use efficiency is often remarkably similar across ecosystems located in different biomes, suggesting that ecosystems organize in a way that maximizes carbon allocation to growth.

Where do plants invest organic compounds designated for net primary production? Consider, as an example, a mature forest. The stems, leaves, flowers, and fruit are all visible displays of aboveground NPP (i.e., growth) that accrued over time — but what about belowground (root) NPP? Most of the NPP readily observed aboveground is matched in magnitude belowground by the less visible, but equally important, production of roots. For example, root growth comprised almost half of total ecosystem NPP in a ninety-year-old Michigan forest, indicating that belowground investments in biomass by plants are substantial (Figure 1). The total standing biomass of an ecosystem is a function of cumulative NPP over time minus biomass losses from senescence (i.e., death). In the same forest, stems (including trunks and branches) are the largest fraction of standing biomass, but roots comprise a quarter of the total biomass present in the ecosystem.

Measuring Gross and Net Primary Production

Figure 2: Meteorological towers like this one located in a temperate forest are distributed across ecosystems in all continents except Antarctica, providing assessments of carbon uptake by forest, grassland, desert, and crop ecosystems.

© 2011 Nature Education All rights reserved. 

Scientists use several complementary tools for quantifying terrestrial gross and net primary production at ecosystem to global scales. On-the-ground inventory based methods are commonly used in cropland, grassland, and forested ecosystems to measure NPP. This approach requires estimates of biomass production through periodic measurements of root, stem, leaf, and fruit growth. The growth over time of all plant tissues within a terrestrial ecosystem is equal to NPP. In this approach, aboveground (ears, stalks, leaves) and belowground (roots) corn biomass yield over a single growing season is equal to annual NPP of this crop ecosystem.

Recent technological advances also allow for on-the-ground estimates of terrestrial primary production using meteorological towers that measure the uptake or emissions of CO₂ by ecosystems (Figure 2). Meteorological towers measure net ecosystem CO₂ exchange (NEE), which is equal to GPP minus ecosystem respiration or the quantity of CO₂ respired by both autotrophs (plants) and heterotrophs (primarily microbes). GPP and NPP are calculated indirectly by adding ecosystem and heterotrophic respiration, respectively, to NEE. Meteorological approaches are employed worldwide in forest, agricultural, grassland, and desert ecosystems to track terrestrial primary production. For example, the international research network FLUXNET (Baldocchi et al.2001) supports observations of terrestrial primary production on six of seven continents.

At the global scale, satellite data combined with mathematical modeling is essential to providing worldwide estimates of terrestrial primary production. Several approaches have been used, but most notable are products derived from NASA's Moderate-resolution Imaging Spectroradiometer (MODIS), a satellite-mounted instrument that collects surface spectral, or color, data useful for tracking changes in the productivity of terrestrial and marine ecosystems. An example MODIS product is a "greenness" index of the Earth's surface used to estimate terrestrial primary production. Surface greenness and other remotely sensed data collected from space provide coarser assessments of NPP and GPP than inventory and meteorological tower based methods but have the advantage of providing estimates of terrestrial primary production for large areas where ground-based methods are not feasible.

Terrestrial Primary Production Over Time and Across the Earth's Surface

Figure 3: Patterns of terrestrial NPP at different timescales in a temperate forest: Daily net primary production (NPP) changes during the growing season in response to climate variables including solar radiation and precipitation, while the duration of NPP during the growing season (i.e., spring green-up to autumn leaf fall) is largely a function of photoperiod. Annual NPP changes from one year to the next in response to longer-term trends in climate, including shifts in total solar radiation caused by differences in cloud cover from year to year. Decadal patterns of NPP track changes in ecological succession (Gough et al. 2007, 2008).

© 2011 Nature Education All rights reserved. 

Terrestrial primary production fluctuates over time and is closely coupled with physical (i.e., abiotic) and ecological (i.e., biotic) changes that play out over different timescales. On scales of seconds to hours, primary production during the growing season responds to environmental drivers of photosynthesis, generally increasing with photosynthetic photon flux density (PPFD) or the spectrum of solar radiation available to power photosynthesis. At the seasonal scale, terrestrial primary production of boreal and temperate ecosystems is tied to changes in temperature and photoperiod, or day length, (Figure 3) while in tropical regions seasonal precipitation patterns often dictate cycles of high and low primary production. Year-to-year, or interannual, changes in terrestrial primary production are often related to long-term climate variation including prolonged drought and, in some cases, variation from one year to the next in average annual temperature and solar radiation.

Over decades, a period that is meaningful to ecological succession, terrestrial primary production changes in response to shifts in plant competition and disturbance. Consider an abandoned field that undergoes a successional reversion back to forest. Plant communities will assemble during early succession, with fast-growing plants emerging first and because of low initial plant density there will be little competition for resources. As a result, total plant growth in the ecosystem, or NPP, will proceed at an increasingly higher rate for several years. NPP generally levels off or declines once plants start crowding one another and begin competing more intensively for limiting light, nutrient, and water resources (Figure 3). Terrestrial primary production also may change over time in response to natural disturbances such as insect outbreaks, wind, fire, and pathogens that diminish photosynthesis by reducing leaf biomass and causing plant death. Long-term increases in atmospheric CO₂ and nitrogen deposition associated primarily with fossil fuel burning generally increase plant growth over long periods of time.

Terrestrial primary production varies considerably across the surface of the Earth and among different ecosystem types. Terrestrial primary production, both NPP and GPP, vary from north to south (or latitudinally) due to gradients in plant community composition, growing season length, precipitation, temperature, and solar radiation. However, east to west (longitudinal) differences in terrestrial primary production also exist. These spatial differences are illustrated in a map of global NPP derived from NASA's MODIS satellite (Figure 4). For example, there is a precipitous decline in NPP from east to west in middle North America that is largely a function of declining precipitation. NPP generally declines from tropical regions to the poles because of temperature and light limitations. Tropical forests tend to be much more productive than other terrestrial ecosystems, with temperate forests, tropical savannah, croplands, and boreal forests all exhibiting middle levels of primary production (Table 1). Desert and Tundra Biomes, limited by precipitation and temperature respectively, contain the least productive ecosystems. In addition to climatic regulation of terrestrial primary production, disturbance, management, and land-use change (including urbanization) play critical roles in determining spatial differences in terrestrial primary production.

Figure 4: The global distribution of land and ocean net primary production (NPP) estimated from spectral data gathered by NASA's MODIS satellite

Public Domain NASA Earth Observatory.

Tropical ecosystems, because of their high productivity and extensive footprint on the Earth's surface, comprise nearly half of global NPP and GPP (Table 1). Temperate ecosystems and croplands are also a substantial fraction of global terrestrial primary production, accounting for roughly a quarter of global NPP and GPP. Global estimates of terrestrial NPP range from 48.0 to 69.0 Pg (= Petagrams or 10¹⁵ g) C yr^-1, with global terrestrial GPP estimated at 121.7 Pg C yr^-1 or approximately double global NPP on land.

Biome	Global GPP1  (Pg C yr -1)	Global NPP2 (PG C yr-1)	Ecosystem NPP3 (g C ha-1 yr
Tropical forest	40.8	16.0–23.1	871–1098
Temperate forest	9.9	4.6–9.1	465–741
Boreal forest	8.3	2.6–4.6	173–238
Tropical savannah and grasslands	31.3	14.9–19.2	343–393
Temperate grasslands and shrublands	8.5	3.4–7.0	129–342
Deserts	6.4	0.5–3.5	28–151
Tundra	1.6	0.5–1.0	80–130
Croplands	14.8	4.1–8.0	288–468
TOTAL	121.7	48.0–69.0	2377–3561
Table 1: Global and ecosystem-scale estimates of mean terrestrial gross and net primary production for the Earth's major biomes from remotely sensed satellite data and modeling students. 1 Petagram (Pg) = 1015 grams (g). 1. Beer et al. 2000; 2. Melillo et al. 1993; Potter et al. 1993; Prince & Goward 1995; Field et al. 1998; Beer et al. 2010 3. Melillo et al. 1993; Potter et al. 1993; Prince & Goward 1995

Haberl et al. (2007) estimated that nearly a quarter of global NPP is used by humans annually in the production of crops for food and fiber, timber for wood products and paper, and in support of livestock grazing. Humans exert an additional influence on global NPP through fires. Many ecologists are concerned that the rising global demand for biofuels, together with continued human population growth, will increase this already large human appropriation of global NPP to the detriment of ecological food webs and biodiversity.

Terrestrial Primary Production and Global Change

Considerable research in ecosystem ecology centers on understanding how climate change is affecting the primary production of terrestrial ecosystems and, conversely, how ecosystems may moderate changes in global climate by absorbing anthropogenic CO₂emissions. Terrestrial primary production is an important ecosystem service, locking up carbon in biomass that might otherwise exist in the atmosphere as CO₂, a potent greenhouse gas. Recent evidence suggests, however, that terrestrial NPP may be declining in response to global warming and accompanying drought, with Zhoa & Running (2010) estimating a 0.55 Pg, or about 1%, reduction in global terrestrial NPP from 2000 to 2009. Continued declines in global NPP would not only reduce carbon sequestration by terrestrial ecosystems but also compromise food security and disrupt the foundation of food webs.

Summary

Ecosystem ecologists have long been interested in quantifying and understanding what controls terrestrial primary production. While gross primary production (GPP) is the total influx of carbon into an ecosystem through the photosynthetic fixation of CO₂, net primary production (NPP) is this gross carbon influx discounted for plant respiratory costs of growth and maintenance. Net primary production forms the base of ecological food chains and is heavily manipulated by humans in the production of food, fiber, wood, and increasingly biofuels. Climate, disturbance, and ecological succession exert influences on terrestrial NPP and GPP, suggesting that mounting anthropogenic influences on global climate and land-use will have substantial effects on the future primary production of terrestrial ecosystems.

References and Recommended Reading

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Baldocchi, D. et al. FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities. Bulletin of the American Meteorological Society 82, 2415–2434 (2001).

Beer, C. et al. Terrestrial gross carbon dioxide uptake: Global distribution and covariation with climate. Science 329, 834–838 (2010).

Field, C. B. et al. Primary production of the biosphere: Integrating terrestrial and oceanic components. Science 281, 237–240 (1998).

Gough, C. M. et al. The legacy of harvest and fire on ecosystem carbon storage in a north temperate forest. Global Change Biology 13, 1935–1949 (2007).

Gough, C. M. et al. Controls on annual forest carbon storage: Lessons from the past and predictions for the future. Bioscience 58, 609–622 (2008).

Haberl, H. et al. Quantifying and mapping the human appropriation of net primary production in earth's terrestrial ecosystems. Proceedings of the National Academy of Sciences USA 104, 12942–12945 (2007).

Melillo, J. M. et al. Global climate-change and terrestrial net primary production. Nature 363, 234–240 (1993).

Potter, C. S. et al. Terrestrial ecosystem production - a process model-based on global satellite and surface data. Global Biogeochemical Cycles 7, 811–841 (1993).

Prince, S. D. & Goward, S. N. Global primary production: A remote sensing approach. Journal of Biogeography 22, 815–835. 1995.

Roy, J. et al. Terrestrial Global Productivity. San Diego, CA: Academic Press (2001).

Zhao, M. S. & Running, S. W. Drought-induced reduction in global terrestrial net primary production from 2000 through 2009. Science329, 940–943 (2010).

2819. Why a Big Utility Is Embracing Wind and Solar

By Justin Gillis and Hal Harvey, The New York Times, February 6, 2018

Turbines at the Spring Canyon Wind Farm outside Peetz, Colo. The farm is owned by Invenergy, and produces energy under contract to Xcel Energy. Photo: Ryan David Brown for The New York Times. 

DENVER — Imagine planning your next trip and finding that Delta was selling first-class seats for less than the cramped middle seats in the back of the plane.

So you fly first class to New York and walk into the best French restaurant, only to discover that every dish is cheaper than the burger and fries down the street. Waiter, bring the duck à l’orange!

Fanciful as that might sound, something a bit like it is happening right now in the world of electricity.

Xcel Energy is a utility company with millions of electric customers in the middle of the country, from Texas to Michigan. In booming Colorado, the company asked for proposals to construct big power plants using wind turbines and solar panels.

The bids have come in so low that the company will be able to build and operate the new plants for less money than it would have to pay just to keep running its old, coal-burning power plants.

You read that right: In parts of the country, wind and solar plants built from scratch now offer the cheapest power available, even counting old coal, which was long seen as unbeatable.

Xcel, Colorado’s biggest power company, has pitched a plan to regulators that will involve replacing two large coal-burning units with renewable energy and possibly some natural gas. The company expects to save tens of millions of dollars as a result. Power bills in Colorado have been falling recently, and they are likely to fall further with this plan.

So the plan will be cheaper, but why will it be better?

Because it will cut Xcel’s emissions. That includes the carbon dioxide that is warming the planet, of course, but it also includes other pollutants, like the fine particles that can send children to the hospital with asthma attacks.

Under the leadership of Benjamin Fowke, a gray-haired chief executive trained in finance, Xcel intends to get far ahead of the clean-power requirements that have been imposed by its regulators.

Across its eight-state system, Xcel predicts that well over half its electricity will come from renewable sources by the mid-2020s. It will be one of the cleanest large utility companies in the country.

Now, to be clear, the low bids that Xcel is getting include some federal subsidies for clean power. Those subsidies are entirely defensible, but both parties in Congress have agreed to phase them out in a couple of years. Mr. Fowke is jumping now in part to lock in the subsidized prices.

Yet costs for renewable technologies are coming down so much that by the time the federal subsidies expire, wind turbines and large-scale solar arrays will still be competitive in large parts of the country.

The same trend is occurring all over the world, even in countries that do not offer subsidies, with renewable projects routinely beating fossil-fuel projects in countries like Mexico and India. We are confident more price declines are coming.

The costs of huge batteries are also falling, and it looks as if they will turn out to be a big help in managing the variability of wind and solar power. Xcel is already testing a battery project near Denver, and it may buy more batteries as part of the new plan.

How, exactly, did the cleanest energy technologies get on a path to become the cheapest?

In a way, the story is as old as Henry Ford and his Model T, or in more recent times, the amazing progress of computer chips.

As they scale up, new technologies often follow a “learning curve” that cuts the cost. But it’s not automatic. You have to build more and more units to drive the prices down.

That happened naturally with consumer products like Model Ts and cellphones, since everybody who saw the things wanted one. But the electricity system was a hidebound, monopolistic industry that used to spend virtually nothing on innovation.

For decades, utility executives who were wedded to coal regarded solar panels and wind turbines as expensive trinkets. But some farsighted political leaders saw the potential as early as the 1970s.

President Jimmy Carter was one. Jerry Brown, then serving as California’s youngest-ever governor, was another. Republican leaders in windy states, like Terry Branstad and Charles Grassley of Iowa, also got on board.

A combination of state clean-power mandates and federal subsidies helped to increase the market, as did similar policies in Europe. It has taken a couple of decades, but we are reaching a point where the new energy technologies are going to be cheap enough to drive a lot of the old coal-burning power plants off the market.

Nowadays, of course, the Trump administration is trying to take the country backward. It recently offered a scheme to subsidize coal and nuclear plants, but the plan was so ludicrous, a federal panel dominated by Trump appointees voted it down 5-0.

More ominously, the administration recently imposed costly tariffs on solar panels made in China. That is unquestionably bad for the solar industry, but we think it will turn out to be a temporary setback.

The real question now is how fast can the fossil-fuel plants be shut down. Even with favorable economics, human and institutional inertia is such that the remaining coal plants could take a long time to die.

States need to find ways to help utilities make the right decisions, perhaps by sharing some of the short-term costs of the shutdowns. They also need to protect workers who lose their jobs, and compensate communities that stand to lose part of their tax base.

Despite such concerns, the cost trends are clear, and inexorable. Mr. Fowke has positioned Xcel to take advantage of them, and a handful of other power companies across the country are taking similar steps.

But most utilities are still doing only what governments have required of them. With the best power plants becoming the cheapest, isn’t it time for their leaders to seize the future, too?

2657. Heinberg on Jacobson vs. Clack Debate on Energy Transition

By Richard Heinberg, Resilience, July 11, 2017

Richard Heinberg

A heated debate in the pages of one of the country’s most renowned scientific journals has gained national attention. The debate is over whether a combination of wind, solar, and hydroelectricity could fully power the U.S. But both sides of the debate are completely missing half of the equation.

In a series of papers published over the last few years, Mark Jacobson of Stanford University (along with co-authors) has offered a series of transition plans for achieving a 100 percent wind-solar-hydro energy economy. These include comprehensive blueprints for the United States, for each individual state, and for the world as a whole. His message is clear: such a transition is not only possible, it’s affordable—cheaper, in fact than maintaining the current fossil fueled system. There is no technical or economic barrier to an all-renewable future—only a political one, resulting from the enormous influence of fossil fuel companies on Congress and the White House. Jacobson’s plans have been touted by celebrities (Leonardo DiCaprio and Mark Ruffalo) and at least one prominent politician (Bernie Sanders).

However, during the past two years, a group of scientists unconvinced by Jacobson’s arguments has labored to craft a critical review of his plans, and to get it published in the same journal that printed Jacobson’s own most-cited paper. They voice a concern that the growing popularity of Jacobson’s plans could lead to critical mistakes in policy making and investment choices. The lead author, Christopher Clack, and his 20 co-authors attack Jacobson’s assumptions and highlight what they call serious modeling errors. Much of their criticism has to do with Jacobson’s ways of getting around solar and wind power’s most notorious drawback—its intermittency. Jacobson says we can deal with cloudy and windless days by storing energy in the forms of underground heat and hydrogen. Clack et al. point out that doing so on the scale Jacobson is proposing is unprecedented (therefore, we really don’t know if it can be done), and also argue that Jacobson made crucial errors in estimating how much storage would be needed and how much it would cost.

The stakes in this controversy are high enough that the New York Times and other mainstream media have reported on it. One pro-renewables scientist friend of mine despairs not just because of bad press about solar and wind power, but also because the reputation of science itself is taking a beating. If these renowned energy experts can’t agree on whether solar and wind power are capable of powering the future, then what are the implications for the credibility of climate science?

Jacobson and colleagues have published what can only be called a take-no-prisoners rebuttal to Clack et al. In it, they declare that, “The premise and all error claims by Clack et al. . . . about Jacobson et al. . . . are demonstrably false.” In a separate article, Jacobson has dismissed Clack and his co-authors as “nuclear and fossil fuel supporters,” though it’s clear that neither side in this debate is anti-renewables.

However, Clack et al. have issued their own line-by-line response to Jacobson’s line-by-line rebuttal, and it’s fairly devastating.

This is probably a good place to point out that David Fridley, staff scientist in the energy analysis program at Lawrence Berkeley National Laboratories, and I recently published a book, Our Renewable Future, exploring a hypothetical transition to a 100 percent wind-and-solar energy economy. While we don’t say so in the book, we were compelled to write it partly because of our misgivings about Mark Jacobson’s widely publicized plans. We did not attack those plans directly, as Clack et al. have done, but sought instead to provide a more nuanced and realistic view of what a transition to all-renewable energy would involve.

Our exploration of the subject revealed that source intermittency is indeed a serious problem, and solving it becomes more expensive and technically challenging as solar-wind generation approaches 100 percent of all electricity produced. A further challenge is that solar and wind yield electricity, but 80 percent of final energy is currently used in other forms—mostly as liquid and gaseous fuels. Therefore the energy transition will entail enormous changes in the ways we use energy, and some of those changes will be technically difficult and expensive.

Our core realization was that scale is the biggest transition hurdle. This has implications that both Jacobson et al., and Clack et al. largely ignore. Jacobson’s plan, for example, envisions building 100,000 times more hydrogen production capacity than exists today. And the plan’s assumed hydro expansion would require 100 times the flow of the Mississippi River. If, instead, the United States were to aim for an energy system, say, a tenth the size of its current one, then the transition would be far easier to fund and design.

When we start our transition planning by assuming that future Americans will use as much energy as we do now (or even more of it in the case of economic growth), then we have set up conditions that are nearly impossible to design for. And crucially, that conclusion still holds if we add nuclear power (which is expensive and risky) or fossil fuels (which are rapidly depleting) to the mix. The only realistic energy future that David Fridley and I were able to envision is one in which people in currently industrialized countries use far less energy per capita, use it much more efficiently, and use it when it’s available rather than demanding 24/7/365 energy services. That would mean not doing a lot of things we are currently doing (e.g., traveling in commercial aircraft), doing them on a much smaller scale (e.g., getting used to living in smaller spaces and buying fewer consumer products—and ones built to be endlessly repaired), or doing them very differently (e.g., constructing buildings and roads with local natural materials).

If powerdown—that is, focusing at least as much on the demand side of the energy equation as on the supply side—were combined with a deliberate and humanely guided policy of population decline, there would be abundant beneficial side effects. The climate change crisis would be far easier to tackle, as would ongoing loss of biodiversity and the depletion of resources such as fresh water, topsoil, and minerals.

Jacobson has not embraced a powerdown pathway, possibly because he assumes it would not appeal to film stars and politicians. Clack et al. do not discuss it either, mostly because their task at hand is simply to demolish Jacobson. But powerdown, the pathway about which it is seemingly not permissible for serious people to speak, is what we should all be talking about. That’s because it is the most realistic way to get to a sustainable, happy future.

2653. What New York Times Got Wrong on Assessment of Transition to 100% Renewables

By Mark Jacobson, EcoWatch, July 9, 2017

This is a response to Eduardo Porter's article in the New York Times on June 20, "Fisticuffs Over the Route to a Clean Energy Future."

Porter's article described a paper published in the Proceedings of the National Academy of Sciences (PNAS) by Chris Clack and coauthors on June 19, criticizing a paper colleagues and I authored in the same journal in 2015. Our original paper showed that the U.S. can transition to 100% clean,  renewable energy in all energy sectors without coal, nuclear power or biofuels. Porter makes several mistakes and omissions in his article that I correct here.

First, Porter relies on his claim that "21 prominent scholars…took a fine comb to the Jacobson paper and dismantled its conclusions bit by bit." This one sentence contains two falsehoods. For starters, our conclusions were not dismantled et al. Our response, which PNAS published as the last word by not permitting a response by Clack, concludes instead, "The premise and all error claims by Clack et al. about Jacobson et al. are demonstrably false. We reaffirm Jacobson et al.'s conclusions."

More important, Porter fails to point out that Clack and coauthors' own disclosure published in their paper indicates that only 3 out of 21 coauthors performed any type of research for the article. The remaining 18 did no research whatsoever, merely contributing to writing the paper with admittedly no research contribution. Of the three authors who did perform research, one has admitted publicly, "I am not an energy expert" (see 15 minutes and 32 seconds into this UCLA debate).

Porter quotes another author, David Victor referring to our 2015 PNAS paper, as stating, "I thought 'this paper is dangerous'," despite the fact that Victor has admitted to doing no research for the article and despite the fact that he is neither a scientist nor an engineer but instead works on international policy and law. Similarly, Porter quotes another author, Varun Sivaram, as stating about the Clack paper, "Our paper is pretty devastating." But, Sivaram has also admitted in writing that he did no research for the article. Moreover, he works in foreign relations, not energy science or engineering.

In the meantime, our 100% clean, renewable energy peer-reviewed papers have collectively had more than 85 researcher-coauthors and more than 35 anonymous peer reviewers.

Porter then mimics Clack's false claim that "most of the scientific community represented on the Intergovernmental Panel on Climate Change" argues that nuclear power is necessary to help solve the climate problem.

However, Porter is wrong. As stated in our PNAS-published response to Clack, the IPCC says the exact opposite: "Without support from governments, investments in new nuclear power plants are currently generally not economically attractive within liberalized markets ..." I don't intend to be harsh. But this statement was in our response, so Porter should never have claimed about the scientific community believing nuclear is necessary in the first place.

Next, Porter claims that I "accused (my) critics of being shills for the fossil fuel and nuclear industries." No, I do not believe any of the authors are shills (someone who is paid specifically to act on someone's behalf), but I do believe most of the authors have either a research, advocacy, or financial conflict of interests in what they have written. For example, one coauthor, Sweeney, "periodically serves as a consultant or advisor to Exxon Corporation, ARCO, the American Petroleum Institute,…", all of whom profit from fossil fuels. He has also stated unequivocally, "If we were to give up on the fossil fuels, we give up on both the economy and security very quickly" (see 1 hour 29 minutes into this video).

Similarly, Jane Long, another co-author, is a Senior Fellow of the Breakthrough Institute, a pro-nuclear advocacy group. It is therefore, no surprise that several authors would criticize our work because of their conflict of interest in keeping fossil fuels and nuclear power on the table.

Porter then criticizes increasing the use of underground storage in rocks, but this storage technology is inexpensive (less than 1/300th the cost per unit energy stored than batteries) and a form of district heat. Sixty percent of Denmark's heat is from district heating using water rather than rocks. Underground rocks are a less-expensive substitute for water tanks. He also somehow thinks it is impossible to build pipes to homes when virtually every new home in the United States has gas and water pipes built to it.

Porter further criticizes the use of more hydrogen, whereas hydrogen production from electricity is an advanced technology developed more than 130 years ago. Porter then unduly criticizes the cost of capital we use and the ability of industry to use demand response to shift times of energy use, when these claims are clearly addressed in our PNAS reply letter at.

Porter then falsely implies that we propose to add new hydroelectric installations equivalent to 600 Hoover dams resulting in 100 times the flow of the Mississippi River. This analogy is nonsensical since our annual energy output is not increased at all, whereas a flow rate of 100 times that of the Mississippi would mean that we would increase the annual energy output of hydropower by a factor of ten, which we don't. The mistake by Porter and co-author of the PNAS article, Ken Caldeira, who provided this claim, is that whereas we increase the ability of the hydro to discharge significantly faster for some hours, we discharge much less during other hours in order to ensure there is zero change in the annual output. Caldeira and Porter tried to make it sound as if we increase the flow rate indefinitely.

Regardless, an alternate solution to increasing the hydropower discharge rate is to increase the discharge rate of concentrated solar power (CSP) and/or to add batteries. Both methods result in low-cost solutions as illustrated for the United States and Canada here.

The fact that the system works with either increased hydropower discharge or increased CSP and batteries contradicts Porter's quote of Clack: "The whole system falls apart because this (hydropower) is the last thing that is used. If you remove any of this, the model fails." To the contrary, the result above demonstrates that the model works without increasing hydropower peak discharge, disproving the main premise of the Clack article that our nation's energy can't run 100% on wind, water, and solar power at low cost.

In sum, I believe that a debate about our energy future can be constructive. But inaccurate statements about scientific work and amplifications of those inaccuracies help no one. Had Porter read our PNAS response carefully, he would not have made the errors he did. Nevertheless, my colleagues and I are always seeking to improve our methods and calculations. Our goals are to better the quality of life of everyone by determining the best ways to provide clean, renewable, and reliable energy while creating jobs and improving people's health and reducing costs. Hopefully, others share these goals, regardless of political party affiliation.