Comment by crazygringo | Hacker News Reader

Comment by crazygringo | Hacker News Reader

This feels misleading.

I totally believe that botanical gardens cool the air within them. That's what happens when you have an area full of trees and shade, with denser vegetation than a park.

But I have a hard time believing that they have any significant effect on the city air 5 or 10 blocks away, where the asphalt is baking in the sun.

So I'm not sure what the point of this article is, because it's not like we're going to replace half the blocks in a city with botanical gardens, as nice as that would be.

Meanwhile, the article claims claims planting trees on the street has less effect, but surely is far more important -- because it affects the whole city, rather than a small localized area in and around a botanic garden?

So there seems to be a major flaw in this article, in that it's comparing the cooling effects of various interventions (botanical gardens, street trees, etc.) but without ever specifying how the sizes or densities are being compared.

Honestly, I can't even imagine what a unit of comparison between botanical gardens and street trees would be, since botanical gardens replace streets and buildings, while street trees merely add to them. It's apples and oranges.

I think the article addresses this fairly well. In addition to shade, evaporation from open water and plant leaves contributes, as does the soil acting as a heat sink.

Botanical gardens are only slightly more effective than trees over roadways from their study, so shade is likely the strongest factor, but the others clearly play a part- from cooling down enough overnight compared to roadways and cement to the evaporation from the denser vegetation having a stronger effect.

The thing that I missed was how such a garden compared to an open, grass park. The difference in vegetation density would be clearer, I think, and might better explain the difference measured between trees over roads and gardens.

From an energy perspective it makes sense, since at least some of the solar energy hitting tree leaves is used for photosynthesis, and reducing Carbon out of its oxydized state. So it's not just accumulated/reflected like for pavement.

Effectively none of the incident energy is used for photosynthesis.

Much bigger effects are reflecting energy well above things that can store heat, and acting as evaporative coolers.

Interesting how easy it is to mitigate 5C - and yet we think the world is going to end if temps increase another 2C - when we are basically in an Ice Age and the Earth has only been cooler for brief periods of time in the last 500M years: https://www.climate.gov/news-features/climate-qa/whats-hotte...

Luckily, fossil fuels are going to get phazed out massively over the next 50 years strictly due to economics.

You are mistaking global for local, and 50 years is too late. I am positive I think some of us will survive.

Did you read the link you posted? Specifically the update at the top?

Easy to mitigate it in urban environments which are heat islands due to the low albedo of man-made surfaces. 99% of the earth, however, is not paved.

Reducing heat on a global scale is a wee bit more difficult.

The increase in temperature is just one of the issues. It has probably been mainly publicized as it's an easy "key performance indicator" to get the point across/that can be succinctly referred to. Sea level rise, ocean acidification, global weather pattern shifts, etc. are all also major problems.

Complete anecdote here, but I live on a park that is about two acres big. Its filled with large old (~100 years) trees and lawns, though it does also cram in a basketball court and two tennis courts. In the summer heat, when we walk around our area, the temperature astonishingly drops about 5 degrees once you get within 2-3 blocks of the park. Its striking in how noticeable it is. I have no idea why, but it seems even a bit of green space can have a big impact.

They do have a significant effect. Trees scoop up rain from the soil, lift it through their trunks and up into the leaves where little mouths (stomata) in the leaves deposit that water back into the air in a process called transpiration.

It is actually in this way that places deep inland can still receive rainfall. Without this process clouds wouldn't be able to make it far inland.

High temperature air rises (by expanding and becoming less dense), the void is filled with low temperature air. So a colder forest will start a wind outward of the forest towards the warmer areas, thereby distributing the colder air into the surrounding area.

You can see the same effect mostly in spring in coastal areas, when the land is heated faster than the sea. Hot air over land will rise, colder air from the sea will move in, causing thermal wind, making the coast a lot cooler. This can cause enough wind for kitesurfing or wingfoiling.

I think it's misleading for a worse reason: These trade temperature for humidity. They seem to work great as long as the temperatures don't go too high. They become hot ovens when they would be the most needed.

Its good to trade temperature for humidity

> But I have a hard time believing that

Have you built your argument against the study on belief?

I don't see anywhere in TFA where it's implied that the temperature drops as an average, or that somehow it extends past the green area. I feel like you've been misled by a strawman that you created yourself.

Empirically, it extends a tiny bit past the green area.

When I go for walks in summer in my city, it's noticeable how the temperature drops while on the sidewalk when I walk past a green area as opposed to past a building.

Thermodynamically it must. High temperature will flow towards low temperature areas like a heat-sink, where it is cooled by the shade and vapour.

It’s actually the opposite. High temperature air rises (by expanding and becoming less dense), the void is filled with low temperature air. So a colder forest will start a wind outward of the forest towards the warmer areas, thereby distributing the colder air into the surrounding area.

Serves me right for thinking two-dimensionally!

Reply on news.ycombinator.com