Hacker News Reader

Comment by mchannon | Hacker News Reader

This strategy, to get rid of some sunlight, doesn't fix our problem with CO2. In fact, it's likely to make the problem worse.

Our planet has two primary natural methods for lowering atmospheric CO2. One, natural weathering, is not impacted with this strategy.

The other, plantlife, is absolutely affected. If you were to take 10% of sunlight away from Earth, then it logically follows that plants would get 10% less sunlight to convert CO2 into starches and H2O into oxygen. The resource you're trying to constrain, in the name of fighting effects of rising CO2 levels, is the very resource you need in order to fight rising CO2 levels.

Sure, blocking or deflecting some light might lower Earth's temperatures temporarily, but you'd exacerbate CO2 levels semi-permanently.

We, as a community, need to come to the realization that our CO2 problem is not strictly a temperature problem.

View on HN

Every idea for geoengineering as a solution to climate change is insane. They're either nonsense, like solar chimneys, or they're like this one: way worse than the problem currently is, and WAY worse than not emitting CO2 would be. Take atmospheric sulfate aerosols[1], which I often see touted as the solution, usually by conspiracy theorists. The kind of people who think "big green energy" is money play, and this simple solution is being hidden for some grand reason.

"Atmospheric sulfate aerosols" means spraying sulfuric acid into the air. It's all the joy of this solution with the added bonus of primarily blocking visible light, destroying the ozone layer, and scattering light so that solar panels and plant life don't work as well. Oh yeah, and SPRAYING ACID INTO THE AIR. Intentional acid rain, forever. It's an incredibly foolish, destructive idea that people with zero drive to do basic research support vocally.

The cheapest, easiest and safest way to fight climate change is to stop emitting CO2. It would be so incredibly easy to stop 80% of anthropogenic emissions. These geoengineering plans are worse in every way.

[1]: https://en.wikipedia.org/wiki/Stratospheric_sulfur_aerosols

> It would be so incredibly easy

I don't think it's as easy as you think it is to cut emissions by that much, and it certainly wouldn't be humane.

Biological sequestration is closer to a real solution, since it is analogous to how the hydrocarbons we're burning were created in the first place. It might be easier to irrigate the Sahara and flood it with Salicornia and salt mangrove than to stop China from burning coal.

>I don't think it's as easy as you think it is to cut emissions by that much, and it certainly wouldn't be humane.

No, seriously, it really is that easy. The average cost of a new car in the US is $30k, the model 3 is 35k. Every single passenger road vehicle could be replaced with an electric car for ~$1 trillion. It would cost even less to upgrade the grid to full battery-backed solar power and replace gas heating with electric. Over ten years thats 200 billion annually to eliminate 85% of the co2 released by the US and 50% of the methane.

In terms of threats to the country that barely registers as spending. The iraq war was 10+ trillion dollars, we shipped 12 billion in $100 bills on pallets and lost it. Renewable energy is near price parity to fossil fuels, even if you adopt it stupidly it still isnt that expensive.

You make china stop burning coal by buying them solar panels. Nobody says no to free electricity.

Your comment equates cost with ease, and obviously that's not necessarily going to be the case. Eg something might be relatively cheap but politically difficult.

Well yeah, thats why its in the subjuctive. To express that if we would collectively stop chewing our cuds, it would be cheap and simple to do those things. My intended point is about the simplicity of scaling production, the abundance of the necessary resources, and the low cost of existing technology.

In multiple comments, you've point blank said it would be really easy. If it's really hard to get the political will to spend the money to do the technically cheap and easy thing, then actually the task is hard.

Political will is not an engineering complication. It applies to any solution to global warming. Im comparing cutting co2 to other solutions, not as a problem in general.

My frustration comes from seeing cutting co2 as somehow harder than seeding chemicals into the atmosphere over the entire planet, or building a tube to the stratosphere, or lanching a sunshield into space. Those solutions are an insane level of difficulty above cutting co2.

> Political will is not an engineering complication

Yes. Both need to be addressed.

I'm not ignoring the political difficulty here, that's specifically what I'm complaining about. Its obviously hard on some level or we would have done it already, like how the ozone layer is being repaired. These unworkable solutions destroy the political credibility of CO2 reduction.

Saying "the problem is easy besides the political component" is not the same as saying the problem is easy. It's an intensely hard problem but the focus needs to almost entirely be on politics, because the rest is easy in comparison. I'm not drawing attention away from the political difficulty by emphasizing how easy the technical side is, i'm drawing attention to the difficulty of the political side.

At some point we as a species are going to have to start taking responsibility for solving the problem or a solution will never happen.

Also, you miss the context of GP's comment by pointing the discussing in this way. They were replying to another comment that claimed cutting emissions was a difficult problem for reasons that are more... physical, I suppose you could say. GP was merely pointing out that technical challenges are not the obstacle holding us in our current predicament.

I don't disagree with you in that the real challenge we face is our species' collective insanity, but I hope you see that in asserting this point that we are both in agreement with GP here.

> Also, you miss the context of GP's comment by pointing the discussing in this way. They were replying to another comment that claimed cutting emissions was a difficult problem for reasons that are more... physical, I suppose you could say. GP was merely pointing out that technical challenges are not the obstacle holding us in our current predicament.

I don't agree with that. I don't see anything in the comment indicating that they saw the problem as anything other than easy to deal with. Have a look at the reply they made to me, where they said "if we would collectively stop chewing our cuds, it would be cheap and simple to do those things". That's still trivialising the social/political component, making it out as if somehow it's just a matter of making a choice as a society. You can't just wish for society to change its attitude, and while it's easy to talk about making such changes, there's the very difficult process of actually making those changes happen.

You might as well say "if we would collectively invent an infinite, completely clean source of energy, it would be cheap and simple to solve the problem." Well, sure. But that's just stating a goal. It doesn't mean it's easy to reach that goal.

The space sail idea is already designed for a scenario in which global warming becomes an even clearer and more urgent existential threat, causing countries to get together and devote a trillion dollars and a team of scientists to a radical solution. Given that context, I think it's reasonable to point out that in a scenario with money and collective will, we should stop polluting, whatever other steps we take.

You think electricity is not made with CO2 ? no country is 100 percent nuclear or solar and batteries are not made using a clean process either. There is absolutely no free lunch and pretending thIngs are easy is ignorant at best.

I said converting the grid to battery-backed solar. Batteries are clean. Cobalt is a heavy metal but when mined sustainably and built with renewable energy the pollution from a producing a battery is very little.

There are plenty of countries whose power is 100% or close to it "made without CO2", usually with help from hydroelectric (e.g. Norway). Solar panels are becoming cheaper and cheaper and cheaper. The situation is closer to free lunch than you imply!

The making of a new car is an extremelly CO2-intensive process. Ecologically damaging, too.

The most green car is the one you already have, as long as you do, even if it's a SUV.

Over medium time scales people switching to smaller vehicles won't really show up as additional demand and thus won't drive any increase in supply.

If people downsized in large numbers it might even move some buyers away from new large vehicles (if the anticipated resale value of large vehicles dropped).

replacements over ten years means replacing cars at end of life. Additionally, 90% of a car's total life cycle CO2 emissions are due to fuel use[1], so its actually very quickly worth replacing a car. With the current grid it's between 30-60k miles, with a fully green grid it's as low as 7k miles.

[1]: http://www.ucsusa.org/clean-vehicles/electric-vehicles/life-...

But that might damage the Amazon rain forest.

http://www.techtimes.com/articles/35172/20150225/how-the-sah...

The cheapest, easiest and safest way to fight climate change is to stop emitting CO2

Which is absolutely true, but no one (basically) is doing it. And the U.S. is moving in the right direction in some metrics (solar, wind) but the wrong in many others (urban planning (https://www.vox.com/energy-and-environment/2017/8/22/1617782...), nuclear (https://www.technologyreview.com/s/603647/meltdown-of-toshib...), and politics (Trump, Cruz, etc.)).

So while I agree with the proposed solution, no one else does, and even then we have to deal with China and India joining the developed world and what that means for CO2 emissions.

So while I agree with the proposed solution, no one else does, and even then we have to deal with China and India joining the developed world and what that means for CO2 emissions.

Don't forget Africa!

>even then we have to deal with China and India joining the developed world and what that means for CO2 emissions.

Evidence disagrees. Chinese coal consumption is declining ever so gradually (~1% annually) despite their economy growing as normal. In fact their overall co2 emissions are slowly falling as well. Its an open question whether this is going to continue or if this is just a false peak, but if their commitment to renewable and green energy is a predictor then they may fall below the us emission level relatively soon.

Serious question: how reliable are the figures from China? How can we trust they aren't fudging the numbers, either at the local level or the national level?

What is there to gain from fudging these numbers?

Continued economic growth at the increased rates that non-renewable energy sources provide, without the external disapproval/sanctions that come with them.

Its based on energy use etc and I believe satellite spectrometetry, so there are massive margins of error but the general trends are pretty hard to hide.

> Which is absolutely true, but no one (basically) is doing it.

The UK continues to meet its reduction targets set in Kyoto in 1990, and its reductions are so massive that it now uses less CO2 per capita than it did in 1900. The UK also has had one of the highest per capita economic growth rates over that time period in the developed world, higher than the US for example. France really has no need of cuts, because its per capita CO2 emissions were already so small (about 4/5th lower for each individual than in the US or Australia). This blanket statement is incorrect.

What about animal agriculture?

agriculture isn't even the majority of methane production even in the US (which has as many cattle as China, but 4.5x fewer people). The single largest source of methane is from natural gas leaks.

Enteric fermentation is still a big problem, but it's a problem for another day- the only reason to solve it now is if it truly is as simple as feeding cows seaweed. That seems highly suspect but I have yet to see anyone accuse it of failing the sniff test.

I agree with the sentiment but your last paragraph needs some (serious) backing "incredibly easy to stop 80% of anthropogenic emissions", really?

I think he means its easy from a technology standpoint. There are still a ton of politics at play here.

We know how to build solar, wind and nuclear plants, do that and just shut down coal power plants. That is like half. Then keep working to get old inefficient cars off the roads. Tax SUVs and give tax breaks to electric buyers.

None of these things are politically easy, but technologically require nothing new and might leave us with a stronger more economy because of the amount of new players and innovations in the fields affected.

But the current political discourse is functionally stalled on the issue because people don't care until it unambiguously affects them. That works great for many smaller issues but for this by the time it affects the average voter we are fucked.

This is a pretty naive view. There are no electric replacements for some of the big consumers of oil (trucking, shipping, airlines, construction equipment, etc).

The parent did say "That is like half."

Electrical emissions are more like 30% according to this[1]. Transportation is another 27%, electrifying half of that would get us close to like half.

We're a long way from any of that, but maybe it's technically feasible with what we have now.

1. https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emis...

Trucking can easily be electrified and the majority of transport related co2 including airplanes, trains, and cargo ships is from personal light road vehicles. Cargo ships are less than 2%, trains are less than 1%. Airplanes are around 7%. Around 30% is trucking. Construction vehicles are all electric/hydraulic already but more importantly are an insignificant source of co2.

Long distance semi tractors drive 500-800 miles a day. That doesnt require a huge battery.

>Long distance semi tractors drive 500-800 miles a day. That doesnt require a huge battery.

You're joking right? Do you realize how much weight they tow compared to the weight of a Tesla? On top of that most of the Tesla weight is the battery itself.

Battery powered semis for long haul is absolutely not a solution.

see my actual math on the subject: https://www.reddit.com/r/teslamotors/comments/4u0yci/why_i_w...

>On top of that most of the Tesla weight is the battery itself.

That is very, very wrong. According to wikipedia the Model S' battery weighs 540 kg, while the car weighs 2,200 kg.

There are already plans in various places to equip highways with overhead wires that will allow trucks to operate like trams for the duration.

Never mind that diesel engines, used in most heavy machinery, is not as picky about its fuel as a gasoline engine is. As long as it goes bang under pressure, a diesel engine can potentially run on it. what is not needed is to electrify anything, but that CO2 in vs CO2 out sums to zero (or a lot damn closer to it than it currently does).

Electrifying all of the highways and roads trucks drive on would basically be building an entire grid again. That doesn't really fall under the technically feasible category.

We literally did it once. That kind of proves it is feasible.

> There are no electric replacements...

There will be soon enough. And even if there was no way to match liquid hydrocarbon fuels for some applications then we just need to make those fuels from renewable sources.

Indeed. Shipping was left out of the Paris agreement but that's a mistake see http://oecdinsights.org/2016/05/04/carbon-emissions-all-at-s... here

Interesting historial fact: the reason the USA was able to engage the Japanese relatively quick after Pearl Harbor is because the shit bunker fuel ships use doesn't burn easily at all so the ship fuel reserves were left intact.

By my accounting it comes out to 1.7-1.8% of global CO2 and even that article says it's ~3%. That's pretty negligible in the scheme of things

large cargo vessels can easily be powered by nuclear reactors, just like aircraft carriers and submarines already are.

Those wouldn't need electricity, however, if you decide to run them off of some kind of renewable bio-fuel or even just natural gas from renewable sources. The main thing is to switch them off of fossil fuels ASAP, then you can move from there to whatever future tech comes along later.

Reducing emissions is strictly a moral argument that has not been won. We don't have to cut CO2. But we should because it is the right thing to do. All the economic hocus pocus of the last couple of decades has not convinced people of that fact.

The economy makes benefit of cheap fossil fuels. We are shifting away from them, but other forms of energy are more expensive, or they are inconsistent, and usually both. Increasing the cost of fuel and power would mean making everything more expensive. For most first-world countries, we can take the cut. But it would absolutely lead to more poverty and starvation in the third world.

You're being hysterical about acid--- using a scary sounding word, even though it isn't. (You're not far off from the people who think that any chemical they don't understand must be bad, even though they never took an organic chem. class!) Volcanoes naturally put sulfur in the air, so what we'd be doing wouldn't all that different.

> or they are inconsistent

Honestly I think a big change that human industry will have to make over the next century is becoming comfortable with intermittent peak power. Some stuff will happen when it is sunny, or windy, and not otherwise. It's not crazy to say "maybe we should only smelt during peak solar hours", and probably not even crazy to say "maybe our bulk freight should only be moving during peak solar hours".

Once you get rid of the need to do energy intensive stuff at night, the power storage problems become a lot easier.

Or we could invest in a proven carbon free generation technology.

Sure, we should build all the nuclear we can, but we should also build all the solar and wind we can, and we'll still end up with variable peaks.

Domes you can't stop without effectively destroying them? Nah, haven't heard of them.

There is a bunch of options being explored for shorter and longer term energy storage.

One example i ran into recently were concrete blocks that would be heated during periods of excess production, and then the heat converted back into electricity during slumps.

Never mind that smarter, nation wide, grids can allow electricity to flow in either direction depending on where the production and need is.

Yes, and we'll need all of the energy storage we can get, both for applications that do require continuous or on-demand power and for applications which require dense, portable power.

But every transformation you put your energy through between harvesting it and harnessing it has a cost, whether you are converting your electricity between DC and AC willy-nilly, storing it as chemical energy, or converting it to mechanical motion and storing it as gravitational potential. If you have a list industrial application that is not time sensitive, and is running at night from an 80% efficient storage mechanism (effectively, even if off the grid), you can boost your efficiency by 25% by running when it is sunny! Who wouldn't do that?

> Never mind that smarter, nation wide, grids can allow electricity to flow in either direction depending on where the production and need is.

Larger grids are also more vulnerable grids. We should learn from the lessons provided by, e.g. the 2003 blackout in the northeastern U.S. We don't need bigger, more vulnerable, more tempting targets for the bad guys. We need more compartmentalization to limit potential damage and make systems easier to understand and repair.

Its literally sulfuric acid and comes down as acid rain, with all the associated negatives of acid rain. Its bad when volcanoes do it too.

Ever heard of positive feedback? Too late to do it your way.

IPCC disagrees

Why do you trust the IPCC?

Because it is based on an aggregation of publications. The executive summary is approved line-by-line by all of the governments in the UN so it's much tamer than the report would suggest, but the synthesis of global data and studies is quite reliable.

It's all data-driven rather than opinion and while it paints a dark future, there is strong agreement in the scientific community that we have not yet triggered unavoidable positive feedback on a catastrophic scale. Even if we stopped emissions dead right now the future wouldn't be roses and perfume but it's not a nightmare. For the near and medium term the most important factor is still the amount of CO2 we emit, which far outweighs the current feedback loops and the ones that are likely to activate soon.

For instance siberian/canadian permafrost, clathrates and arctic albedo have all been triggered to some extent, but the releases have been outpaced by anthropogenic emissions. This will change, first with arctic sea ice, but at this point we are reasonably certain they won't pick up exponentially until we expect them to.

So what about the IPCC scientists who have whistleblown and quit because of their unscientific practices?

Geoengineering is just that thing we're going to do out of desperation, which will hasten the end, nothing more or less.

If we do it, it will be out of desperation, but it won't hasten the end. It will buy us time while we actively remove carbon from the atmosphere.

It will hasten the end because?

We don't really have any concept of how to do it, and it will be a hammer blow to an already depleted biosphere. the cynic in me also suspects that it would lead to war.

Until the permafrost melts and starts pouring methane and CO2 into the atmosphere, which we can't stop without an equally insane plan

It's sub-sea permafrost and it's possibly we have already damaged it so badly all the methane will leak and there's little we can do and there's little we know -- Russian Artic seafloors are in every which way pretty hostile to scientific research. (And also oil drilling, if you want to know more Google for Exxon Kara Sea and ponder what it means the Russian-awarded ex-CEO of Exxon is now the Secretary of State)

As far as I am aware, most (all?) discoveries were made from surface vessels, often seeing a foaming sea, foaming methane. It's real bad.

It's depressing that there's nothing any of us can do about it. All we can do is hope for some sort of miracle scientific solution. I hate the idea of geo engineering but the alternative seems impossibly worse

> it would be so incredibly easy to stop 80% of anthropogenic emissions.

Please provide some evidence for this massive claim.

80% would be electrifying all road transport and making grid electricity renewable, and converting processes to electricity (primarily converting home heating). Road+heating+residential electricity is about 70% of co2 in the us and 50% of methane. The remainder is industrial use of electricity.

NB that this claim is in response to a proposed solution of building a gun to shoot a sun shield into space. The difficulty is relative. One is incredibly easy compared to the other.

...and where would the electricity come from?

Electricity isn't an energy source. Right now the US generates ~65% of electricity from burning coal and natural gas[1]. This is a common misconception with hydrogen fuel cells as well, which are chemical batteries, not energy sources.

While I agree with your general sentiment in this thread, simple electrification isn't enough; we also have to roll out major renewable energy generation and storage (which is still far wiser than building a gun to shoot a sun shield into space).

[1]: https://www.eia.gov/tools/faqs/faq.php?id=427&t=3

mistyped and left out 'renewable' grid electricity. In my defense, I have norovirus right now and my brain is currently flowing towards a water treatment facility somewhere.

Even converting all of those sectors to electric/renewable, you don't remove all greenhouse gases, you cut them down. In the case of personal transportation we see estimates of ~53% reduction in carbon footprint. I don't know what the improvements would look like if all utilities were forced to go renewables but we would certainly see large increases in price.

This doesn't seem easy at all.

You're criticizing that solution based on the name instead of based on its flaws and then you proceed to make absolutely baseless claims like the one about the acid rain, which completely disregard that the amount of sulphites needed is far less than what we already dump into the atmosphere on a yearly basis.

Please do more research before stooping down to sensationalism and fear-mongering.

> which completely disregard that the amount of sulphites needed is far less than what we already dump into the atmosphere on a yearly basis.

Nonsense. The cooling is less effective at higher altitudes, and all the sulfate currently in the atmosphere provides a forced cooling of .4 W/m^2 vs the greenhouse gas warming of 2.4 W/m^2. The cooling relies on atmospheric water which decreases the higher you go.

We would need to dump way, way more sulfates/sulfites into the air than we do currently.

Amount of sunlight is not the limiting factor on plant growth in most places. In fact, it's often the opposite - plants shut down respiration during midday to limit water loss because it's too hot.

I imagine "most places" refers to most places on land, which is less than 20% of the range of where plants grow. Ocean-based plantlife is the workhorse on this planet, and I would argue that the lion's share is absolutely constrained by available sunlight.

Further, there's a substantial difference between "respiration" and absorption of sunlight. Pineapple plants, for instance, only "breathe" at night.

Plant life in the oceans is mainly limited by minerals, especially iron. That's why the oceans aren't green and slimy. Dumping iron powder in will usually cause an algae bloom, which eventually die and take the carbon they fixed to the bottom, so seeding the oceans with iron is a potential way of reducing CO2 in the atmosphere. Of course, like any large-scale meddling, there will be collateral damage.

> will usually cause an algae bloom, which eventually die and take the carbon they fixed to the bottom

It works so well that this could even be how the current "icehouse earth" conditions got started in the first place. The Azolla bloom of 49 ma dropped atmospheric CO2 levels from 3500ppm to 650ppm in less than 1 my, and CO2 has been in that lower range ever since.

https://en.m.wikipedia.org/wiki/Azolla_event

A quick, non-biologist's googling suggests that nitrate and ammonium are the limiting factors for most oceanic algae.

I recall there being experiments for areas are iron limited, so they spread iron sulfate and created algae blooms.

I suppose intuitively, if you scoop up a bucket of ocean water, it is not green. If light were the limiting factor you would expect the water to contain lots of light absorbing algae. This does happen in nutrient rich waters, like the sewage lagoons and creeks I grew up near.

You can't assume that a reduction in sunlight reaching the top of the atmosphere means a reduction in total sunlight reaching plants on the earth's surface, because of complex feedback effects involving cloud cover.

Moreover, as multiple posts in this thread point out, sunlight is not the limiting factor in plant growth.

Maybe the diffractive elements the paper proposes could deflect wavelengths not used by plants. This would reduce total insolation on Earth but leave photosynthesis and CO2 absorption untouched.

There isn't going to be one single solution, but reducing incoming sunlight by a small percentage (1-2%) could be a cost effective part of a multi-layer approach.

Look, even as a scientist it is incredible hard for me to have a solid opinion on the CO2 influence. (Is it causing warming? Is warming a good or bad thing? etc.). But climate change is the rule in earth history, not the exception. But I kindly ask you to have a look at this link: https://www.nasa.gov/feature/goddard/2016/carbon-dioxide-fer...

You could definitely reflect ex. infrared without affecting photosynthesis. I think your response is a little quick to brush off these ideas. There are many possible venues for geoengineering that could have fantastic benefits, even beyond affecting climate change. Brushing them off as ideas akin to "playing god" is both unprincipled and potentially harmful to both mankind and the planet.

Sunlight is not the bottleneck for plant growth, CO2 and temperature are.

>The other, plantlife, is absolutely affected.

Astute observation, but the problem is easily fixable. Plants use particular wavelengths of light (IIRC mostly from the visual spectrum) for photosynthesis. If the proposed spacecrafts were to block mostly UV and infrared, letting most of visible spectrum through, it'd do plants no harm at all.

The relevant technology is already available and widely used. For example typical car's windshield consists of two or more layers of special UV/infrared absorbing foil sandwiched between glass panes for structural support.

In case of a spacecraft, all you'd have to do is spin the fuselage at low RPM to keep the foil extended.

EDIT: my only contention with any sun-blocking spacecrafts is that they'd have to be carefully engineered to prevent use as a weapon for attacking ground & orbital targets with focused beams of light.

Re: holy shit has that idea ever been explored? Giant lens / mirror array in space?

> Sure, blocking or deflecting some light might lower Earth's temperatures temporarily, but you'd exacerbate CO2 levels semi-permanently.

This hurts to read.

Blocking sunlight "might" not lower temperatures. It is absolutely certain. It's a very basic fact that sunlight is what heats our planet.

I think you're misreading. The uncertainty of "might" refers to the plan actually occurring, not to the temperature lowering.

You can read this sentence as:

"Sure, you might block or deflect some light and that will lower Earth's temperatures..."

That beings said (though, I am CERTAINLY no proponent of this idea), were we to make this geostable "sunshade" out of some sort of solar power generation device (since all of this is ridiculous conjecture, I don't have to think about the logistics) it could cool the (already) elevated global temperatures while reducing green house gas emissions. Though, that would create it's own long term issues to be solved, but all of these fanciful odeas tend towards band-aids, instead of solutions =) still, fun thought experiments

This one for one ratio that you mention of blocking the sun out would not logically follow. Any plant life losing sunlight that does any significant amount of carbon absorption would die. Where are they planning to throw some shade Antarctica?

What's the point of lowering CO2? Isn't the only reason to try to lower CO2 is because it is acting as a GHG?

Ocean acidification, for one

https://en.wikipedia.org/wiki/Ocean_acidification

> If you were to take 10% of sunlight away from Earth

No one is talking about 10%. Article mentions 1.8%.

Reply on news.ycombinator.com