According to a new report from Bloomberg New Energy Finance, global investment in clean energy is down 14% from last quarter, and down 20% from a year ago. And yet, in 2012, we saw the second-highest rise in CO2 emissions on record. Meanwhile, the Asian Development Bank predicts a 67% energy rise from the Asia Pacific region, and coal is likely to be its main driver.

In other words, less cash, more CO2.

So if we need clean tech investment now more than ever, why are investors wasting time on photo apps and startups that steal your mail?

Past failures is one reason. At one of our PandoMonthly events, KPCB investor John Doerr said, “I encouraged us to do too much green too fast.” And indeed, clean tech startups require a huge amount of investment devoted to research, development, and prototyping before a viable commercial product is even possible. Yet with investment rapidly falling, we could use a little overzealousness right now.

There are also regulatory concerns of course, but there are always regulatory concerns, whether you’re a taxi app or a high-altitude wind startup. If Uber’s Travis Kalanick is willing to fight the power for his libertarian fleet of blackcars, why can’t we do it on behalf of something noble?

Or maybe, despite the overwhelming data begging us to cut back on fossil fuels, people just don’t care enough. In the Bloomberg report, BNEF chief executive Michael Liebreich says, “Governments accept that the world has a major problem with climate change but, for the moment, appear too engrossed in short-term domestic issues to take the decisive action needed.” The bad press given to floundering high-profile startups like Solyndra certainly doesn’t help the public image of cleantech either.

But despite these troubling trends, there’s still a lot to be excited about in clean tech. Yes, 2013 has been tough on some major solar companies, but solar installation itself is way up. Electric cars have begun to enter the mainstream; so much so that the Tesla Model S won Motor Trend’s Car of the Year award. And there are startups experimenting with bold new technologies like high altitude wind power, enhanced geothermal systems, and in-pipe hydropower.

Below we’ve outlined nine alternative energy sectors, along with the environmental, economic, and regulatory pros and cons of each. We won’t sufficiently reduce emissions using just one of these techniques. And maybe some of them will prove to be more trouble than they’re worth. But this isn’t the time for sure things and safe bets.

Too bad we don’t have five more Elon Musks.

What is it?  Photovoltaics, aka solar panels, converts radiation from the sun into electricity using panels made of a solar conductive material like silicon or cadmium.

Good news: Since 2011, the cost of solar power has dropped 60% even as researchers continue to hit new milestones in improving the efficiency of these systems. In other words, we’re paying less for a better product. And although most commercial panels only have an efficiency of 15-16 percent, the amount of energy found in the solar radiation that hits the Earth far outweighs the global energy consumption. From a consumer perspective, using solar will reduce your electric bill and maybe even result in a cash credit once per year from the electric company. And the development of battery-backed systems allows customers to store unused solar power for use at night so they may not need to rely on the electric grid at all. Oh, and the sun is free.

Bad news: Although solar panels save money on your electricity bill, the systems cost between $20,000 to $30,000 to install in a typical home. Sure, over time the savings to your electric bill will counteract the upfront costs, but the high barrier of entry has limited the growth of solar adoption. (One solution that holds promise is the rise of solar leases offered by companies like SolarCity, where a company owns and pays for the system, then charges the customer a monthly energy fee that ends up being lower than an electric bill would be).

As for the solar industry itself, many companies have fallen on hard times, most notably China’s Solartech which went from the largest producer of solar panels to the verge of bankruptcy in just two years. But observers like Technology Review’s Kevin Bulls argue this is a simply a natural shakeout following a period of booming investment and government subsidies that have since settled down. The boom helped drive prices down and increase adoption, but the production capacity of the solar industry far outweighs the current demand, so not every company will (or should) survive.

What is it? A new method for extracting heat energy stored in the Earth that doesn’t require the conditions normally needed for traditional geothermal systems

Good news: An MIT study found that with a reasonable investment of R&D, geothermal energy could create the same amount of energy as 100 large coal plants by 2050. But traditionally, to extract geothermal energy from the Earth you needed a very specific set of conditions including naturally occurring heat, water, and rock permeability below the Earth’s surface.

Now, a relatively new method pioneered in part by AltaRock (which has raised $26 million from Google, Kleiner Perkins, Khosla Ventures, and others) can extract heat from dry impermeable rock, where most the geothermal heat is found, by injecting high-pressure cold water which shears the rock and picks up its heat. The hot water is returned to the surface where it is used to power a generator. The method is not unlike the one used in hydraulic fractured gas drilling or “fracking,” however enhanced geothermal systems (EGS) do not use potentially-hazardous chemicals. Nor is there a risk for methane leakage because the system extracts only heat, not gas.

Bad news: Like all experimental technology, it takes a lot of time and a lot of money before any big strides are made. In 2007, AltaRock suspended a $17 million geothermal drilling project in California. The company also faced protests over “micro-earthquakes” caused by fractured drilling, and whether those could lead to more substantial quakes. In 1996, during one of the earliest experiments with EGS, a company called Geopower Basel drilled in Basel, Switzerland. The project created micro-earthquakes as expected, but also increased seismic activity at the surface. A few hours after ceasing operations, Basel experienced a 3.4-magnitude earthquake.

That said, Basel has a history of earthquakes and was the site of the biggest earthquake to ever hit central Europe. AltaRock CEO/founder Susan Petty tells GigaOM that they did a careful and ongoing analysis of the seismic activity of their newest EGS site. She says trains running through the area had higher seismic activity than their project.

What is it? Nuclear power harnesses the energy created when the nucleus of a uranium atom is split.

Good news: In terms of CO2 reduction, nothing beats nuclear power, which by replacing fossil fuel emissions reduced CO2 by 2 billion metric tons in 2005 alone. Indeed, CO2 emissions are on the rise in places like Japan and Germany where nuclear power began to be phased out following the Fukushima disaster. And despite the highly radioactive waste produced by nuclear power, it’s coal power that result in a higher public exposure to radioactive elements because its waste is released at a much higher rate and does not undergo the same rigorous containment procedures. Meanwhile, a startup called TerraPower, which is funded by Bill Gates, is working to improve both the efficiency and waste management of nuclear power by using nuclear waste to power reactors. If successful, TerraPower’s technique would also cut down on the proliferation of nuclear weapons, since the uranium isotope they rely heavily on differs from the isotope used in nuclear bombs and most of today’s reactors.

Bad news: The threat of a nuclear disaster or meltdown is the most visceral reason not to use nuclear. The Chernobyl explosion in 1986 resulted in the deaths of 31 people of acute radiation sickness (ACS), however the International Atomic Energy Agency estimates that thousands have died of fatal cancer associated with that incident. The second biggest nuclear accident occurred at the Fukushima Daiichi plant in Japan following the tsunami and earthquake in March 2011. And while there were no deaths caused by ACS, a huge amount of radiation leaked into the ground and the ocean. (It’s worth noting that a report commissioned by the Japanese government determined that human error was to blame for the meltdown, not the tsunami or earthquake).

The other big problem with nuclear energy is waste disposal. Although companies like TerraPower may discover a way to recycle radioactive waste, currently there is little to do with the highly radioactive material except bury it deep in the ground. This method is hardly fool-proof; for example, leaks at a nuclear waste site in Hanford, Washington were reported last Summer. And while officials say it poses no immediate threat to the environment or public health, the half-life of many isotopes involved in nuclear energy production are on the order of tens of thousands of years.

What is it: A way to harness high altitude winds using kite-like structures.

Good news: Wind power has long been a staple of renewable energy. But winds at high altitudes, thousands of feet off the ground, and in the future perhaps miles off the ground, are much stronger and more consistent than the winds captured by fixed wind towers. Climate scientist Ken Caldeira tells Yale’s Environment 360 website that the power density in the Earth’s jet stream is a hundred times that of sunlight hitting a solar panel. The kites also make it easier to harness winds over the ocean or other places where building a large fixed structure can create problems. These kites can also be lowered, raised, or shifted laterally to find the strongest, most optimal winds to convert into energy. Companies like Makani, which in May was acquired by Google X, are using computers to automate the process.

Bad news: In addition to stronger, more consistent winds, there are a slew of other problems that can arise when operating turbines so high off the ground. The likelihood of damage caused by extreme weather like lightning is greater, and you can't just send a worker up there to conduct maintenance as needed. If high altitude kites can ever harness wind from the jet stream miles above the ground, it will have to contend with commercial airplanes and the regulatory headaches that come with that. Currently the Federal Aviation Administration limits tests of high altitude wind power to 499 feet.

Here’s Calderia again: “I would be reluctant to remortgage my house and invest the money in these companies, because I think the probability of them being able to compete in the marketplace at scale in, say, the next decade is pretty small.”

What is it: An electrical grid that uses information technology to optimize performance and efficiency. It does so by predicting and reacting to the behaviors of consumers.

Good news: According to the Department of Energy, if the electrical grid were just 5% more efficient, it would equate to permanently eliminating the fuel and CO2 emissions of 53 million cars. Thus the opportunity to make a difference is huge. The challenges are also uniquely suited to today’s technology companies, the goal being to leverage data to transmit electricity more efficiently. It’s not unlike consumer devices that save energy, like the Nest thermostat; dial it down to certain buildings during times of repeated non-use while dialing it up to other parts of the grid. Smart grid technology can also identity and isolate surges and outages before they spread and cause larger problems.

Bad news: Unlike many of the other clean-tech initiatives on the list, the concerns over smart grid implementation are largely ethical. The Electronic Privacy Information Center (EPIC) expresses concerns that such close monitoring of devices in our homes, as facilitated by “smart meters,” poses huge privacy risks.

Others worry that the increased complexity of smart grid technology will allow utility companies to take advantage of customers. Meanwhile, utility companies worry that customer apathy and even antipathy will be a roadblock to implementation. (Watch a homeowner pull out a gun to stop a smart meter from being installed).

What is it: Hydroelectric power turbines that are located in municipal, agricultural, or industrial pipes.

Good news: Clean tech’s all about harnessing energy that goes unused, so what better power source than your city’s water, which already has piping and infrastructure all set up? All that’s needed are the turbines and that’s where Lucid Energy comes in. Lucid can install four turbines over a 40 foot section of pipe, creating over 3 megawatts over one mile of piping. The idea is for cities to use that energy for water treatment, and thus the potential for building sustainable clean water infrastructures is huge. So far the power system has been installed in only one city, Riverside, CA, but Portland, OR and San Antonio, TX are on the way.

Bad news: According to Riverside Utilities Manager David Wright, it takes about five years to recoup the costs of Lucid’s system, but the turbines only last eight years. Until turbines are developed that can last longer, that city is waiting to expand their use.

What is it: Energy derived from burning living or recently-living biological material

Good news: We’ve got plenty of it. Barley, sugar beets, oats, potatoes, rye, corn, pine, coconuts… these are all crops that can be converted into energy. Biomass only makes up about 1.42% of all energy used in the United States. But according to the World Bank it makes up 70% of all energy used in Africa, and some techniques are more environmentally sound than others. Oregon’s Biomass Sustainable Energy Co. has spent millions monitoring emissions and making the process as environmentally sound as possible. Algae fuel is also a huge sector, however it’s struggling to scale over the years. That said, promising things are coming from the San Diego-based algae startup Sapphire, which recently paid back a $54.5 million government loan used to build a 100-acre algae fuel farm (though it should be noted it paid the loan back with private investor funds, not algae fuel profits).

Bad News: Some people like to say biomass is “carbon neutral” because the burned plants merely emit the CO2 they already absorbed. But that’s not always true because it takes energy to grow and burn the material. In fact a reason study found that burning forests for fuel resulting in a permanent increase in atmospheric CO2.

What is it: Okay, so this isn’t exactly an alternative energy source. But a ton of startups and organizations are working to promote local food in an attempt to cut back on transportation costs and, thus, fuel.

Good News: Investors have cooled on cleantech, but that doesn’t mean startups that benefit the environment are entirely on the ropes. Food startups are killing it. And many of them like Good Eggs and Farmer’s Web share cleantech startups’ devotion to sustainability and local food.

Then there’s BrightFarms which builds and operates greenhouses at or very close to supermarkets to cut down on fuel and travel time. At the cutting edge of things, there are vertical greenhouses which would allow huge farming operations to exist in urban centers, cutting the trip from farm-to-plate even further.

Bad News: According to the Wall Street Journal, vertical greenhouses are still producing only a small amount of food, and a business model hasn’t yet emerged. And as for food startups, it’s slightly disconcerting that the only way to get VCs to drop cash on environmentally-friendly startups is through their stomachs.

What is it: Automobiles that rely on electrical energy stored in batteries.

Good news: Back in 2006, there was a movie called “Who Killed the Electric Car?” Now, seven years later, Tesla’s electric Model S is Motor Trend’s Car of the Year. Palo Alto now requires all new homes to be built so an electric vehicle (EV) charger can be installed. And EV sales are projected to grow at a rate of 40% each year for the rest of the decade, while the overall auto market will only increase by 2%. The reports of its death have been greatly exaggerated. Take heed, investors who say “cleantech is dead.”

Electric cars emit no tailpipe exhaust and require no gasoline. And while they do use electricity, in most cases, depending on how reliant your area is on coal, electric cars outperform even the most efficient gasoline vehicles.

Bad news: If you live in an area that depends largely on coal for your electricity, the carbon footprint of your electric car may still be considerable. A Union of Concerned Scientists report found that in some cases, electric cars may not be any better at reducing CO2 emissions than some gasoline models. There are also consumer concerns over high prices and a still-budding infrastructure for charging stations across the country. (though charging stations are on the rise)