I've been pretty happy with this:

Cannondale Base Layer Sleeveless White

Also have one of these:

Louis Garneau SF-2 Sleeveless Top | Competitive Cyclist

Have limited experience with the OP's 'skyrocketing humidity' conditions, however...

If it's the right kind of base layer, quite possibly!!

if i understand some of the arguements here correctly, wouldnt it keep the layer of sweat close to your skin, and the evaporation of your sweat cools you down even more?

If you are perspiring your body needs cooling. It doesn't matter what season it is or what the ambient temperature is. It's almost always better to have the perspiration evaporate as quickly as possible.

yep i agree if you are sweating that means you need to cool down anyways

the question was: will it also keep you warm in the winter?
and i guess if you arent sweating then i guess it will, just another layer to trap the air between the jersey.

I have a couple of heavy duty base layers that I wear in the winter, one even has a wind blocker on the chest, I don't think those would work so well in the summer. Of course, I'm usually a sweaty mess at the end of my ride when I wear them in the winter, so maybe.

How does a base layer cool?
As has already been mentioned, it keeps your sweat on your skin where it's evaporation cools your body. Thus such a baselayer must be skin-tight.

Second, imagine a fan blowing air over a large bowl of water. Water is evaporating off the surface, right? Replace the bowl with a wet towel of equal diameter. From which will water evaporate more quickly. The towel, right? Because there's more surface area per measured area.

So a jersey would work just fine, if it were also skin tight, as tight as a UA garment. Perfectly fitted skinsuits are that tight. However, most of us aren't about to wear a jersey that tight, even if we wanted to bother having it tailored for us.

Anyway, that's my reasoning for deciding to try out the concept. $30 for the UA HeatGear LS shirt on Amazon.

FAIL!

So I gave the UA Heatgear a try this Sunday on our first 2-pass ride of the year. It was 85° and a light tailwind on the second pass. About halfway up, I realized I was very dehydrated even though I'd been drinking steadily. I took off the soaked longsleeve UA layer and put on my PI sunsleeves and had a much better time from there.

I think what happened is that the heatgear works by making one sweat even more than usual - another layer, right? But if there's a breeze, one feels cooler because more sweat is evaporating. However on a pass climb it doesn't work that way. It just made me sweat more and didn't cool me. I didn't realize how much water I was losing. After I got that thing off, I kept drinking steadily and by the top of the pass my resting HR was down from 120 to 100 and I felt a lot better.

So maybe they're fine for short flat rides, but I'll never wear it again on a serious ride. This one was 75 miles and 5400', which is very short by rando standards. I went through ~90 oz. in 2.5 hrs., which wasn't nearly enough but all I that I had, from wearing that thing.

I did short and flat...just a touch over 20 miles in 100+ degree heat on Saturday with the UA Heatgear and had the same issue. I felt stuffy and itchy the entire time. I also chugged a gallon of water when I got home.

It seems it would work better mountain biking or with a loose fitting jersey where there would be some air movement around it. With a tight race cut jersey, it just kept me hotter than normal.

User error.

Didn't use the complimentary 'Axe' he-man spray?

Thank you for your suggestion.

Agreed. He shouldn't have worn the extra insulation to begin with.

I'm not a fan of the non-mesh style base layers like the UA when it's hot. Open mesh base layers work much better in the heat since they don't end up acting as insulation.

$85. best cycling decision i've ever made. indispensible.

Agreed. I own probably about 5 different brands of baselayer and the rapha merino wool is by far the most comfortable.
The only downside is the price.

85 degrees? Oh my how did you even make it home in that kind of brutal heat?

Nah, no under shirts. I usually just wear my bibs and a full or 3/4 jersey and by the end of the ride its usually open

You must not live around here. Our normal riding temperatures are from 35°-65°. Still, that was a nice sneer.

Sorry just poking fun. But seriously I've ridden in temps over 110 and pretty much all summer long every summer in temps over 95. Never have I not worn a base layer. Its more about acclimating yourself to heat than anything.

FYI I bought some of these based on your recommendation. They're pretty nice and for $12 a pop, not exactly breaking the bank. My chief complaint is that they're really long: Like basketball shirt long, and I'm someone with a long torso. Also not really very tight. I think I'd get a L rather than an XL next time, and I'm a big guy.

So maybe the takeaway should be that if you're going to be riding in temps 20° higher than what you're used to, more insulation may not be a good idea. Last year the high on this ride was 66° and we were 15' faster, even though we are stronger this year.

Currently 54 degrees & 97% humidity. Afternoon winds of 10-15 mean rapid cooling if you are standing around. Freeze into the wind, boil down wind. Light thermal long sleeve under jersey & light jacket allow for changeout but I always boil downwind. Been debating a light thermal zippable long sleeve over jersey or maybe vest? Of course we also get rain showers this time of year & 5 minutes of that and I'm soaked.

I have a castelli mesh baselayer that I find much more comfortable and helpful than the UA in the heat. The UA was recommended in this thread and is half the price of the castelli baselayer though, so I thought I'd try it. Apparently, whoever recommended it doesn't live in a place where riding in 100+ degree temps is a norm.