Birds and flying bugs would be larger, and/or with smaller wing spans. Most likely new forms of air dwelling critters like atmospheric plankton, plankton that are in clouds living in the denser air mass. Everything with lungs would have either smaller lungs (having more available oxygen per breath) or would be faster/stronger and also live faster - or if you prefer die earlier. It is possible that many things would be larger, like bugs which are constrained in size by the lack of lungs - denser or more atmosphere might very well mean giant bugs - perhaps triple the size they are now days. Storms would be stronger, rains heavier and perhaps longer. Clouds hm hard to say more dense perhaps since the denser atmosphere could carry more mass. Due to the increase in heat weather patterns would be vastly different, definately there would be an increase in water vapor in the atmosphere and evaporation off the oceans, we can assume that cloud formation would take place higher still cooling for rain - perhaps a lot more rain everywhere. In the end it would be difficult to calculate exactly what the earth would be like. It would be alien to us since nothing that we know, plant life, animal life and climate would be the same.

I agree; Any life form that is dependant on oxygen will have less limitation on their growth. I suspect any form of living material that is dependant on air (may it be oxygen or carbo hydrates) will have enormous growth potential. Plants will grow stronger and faster. Organic growth and healing for that matter will increase exponentially. Take in consideration the vast effect the air pressure difference makes on an athletes performance being inland or coastal. Where this difference in air pressure is rather small compared to double athmosperic pressure. I disagree that any life form will die faster. I think rather than their lifespan will double, if not more. Take in considiration the effect on high pressure chambers that is used by top athletes for recovery from sports injuries. These chambers sometimes halves the healing time required. And that is merely for a few hours a day. Imagine the effect of living in such conditions. Take also in considiration the enormous effect efedriene has on cyclists - it basically allows for double oxygen takup for the red bloud cells. Some cyclists do blood infusion, where this also increases the amount of oxygen a person can take up. These give extreme benifits to cyclists and is susbequintly banned. I think that Double atmospheric pressure will not just be vastly different on earth, but will increase the living span and queality of live of all living thing on the planet.

We can look at this from a number of points (atmospheric chambers, scuba diving,sports medicine, high altitude climbing, etc.) the reason I say this is because we can speculate all that we want but in the end we have to use more scientific and historic data. with atmospheric chambers we see one significant change in creatures that the red blood cells can carry more oxygen to the body. Reason, when atmospheres of pressure are in effect molecules usually compress in volume, therefore redblood cells, which carry oxygen on their outer layer, usually are saturated with o2, but when the amount of o2 is smaller the same red blood cell can carry more to areas on the same trip and since the o2 is smaller it is easier to release in smaller areas (like bruises or extremities). scientists have surmised that our planet may have been like this during the time of the dinosaurs, Reason for believing this. Evidently in amber (sap hardened) when a bug gets trapped it does a wonderful job of preserving it. It has been shown, when they drill into it, mosquitos and such have about twice as much atmospheric pressure. 2ndly, animals such as Bracheosauraus have to small of nostril holes and to large of lungs for our atmosphere and would die quickly for lack of o2,(this is a theory, perhaps something else fixed that but best evidence points us in that direction, and third similar animals that live today have been preserved and are simply multiple times larger, (dragon flies, crocodiles, arthropods, turtles) however animals like (horses) seem to have been smaller near this relative time. (this does not mean that these are in reflect to the atmosphere, genetics or something else may have played a valuable roll in this). However the inverse seems to take a roll in low atmospheric areas, such as on top of mountains (k2, or everest, etc.) where climbers have a hard time to move, the difficulty in problem solving, etc. If you do a little research in this avenue it will give some answers as well. So to surmise, if you look at the time of the Dinosaurs to early man (evolutionary theory) or look at the Adamic to Abrahamic era (creationist era theory) you will it may give some good hints. larger animals (dinosuars, crocidiles,turtles,dragon flies, arthropods) ease of physical activities (since we get tired because of a lack of o2 logically you could run a long distance without being in good shape [logically])or hold your breath longer, or move a heavier object. (NO NOT A MONOLITH LIKE STONEHENGE) hahah. but simply an ease of difficulty. you would also "recharge" or get your wind back (similar to a football player when he goes to the bench for just a min. they put him on pure o2 to get him back quick (dont try at home) larger plants, (Japan tomato plant,tree) japan pressurized a tomato plant and as far as I know it came back each year for (8?) years, it turned into a tree, produced an extreme amount of fruit, and produced them faster, and larger (thats all from my recollection). So just as you see in Dino Books, Ferns are huge, average trees look like Giant Redwoods, suppposedly some mushrooms grew to larger than 50 ft.(dont quote me on that, read it somewhere and dont remember where.). generally everything seemed to be larger and stronger and live longer. hope that helped with some avenues. good luck on your world.

It would feel a lot like lying horizontally at [edit] ten meters (33 ft.) under water. The pressure you feel on your body at that depth is two atmospheres.

Well for one thing there would probably be no liquid water on the surface of the planet

I suspect living things might be bigger OR the surface would be void of any water and nothing lives.. or the opposite..all water and we either learn to swim or drown. It would likely be much hotter though and maybe so hot everything eventually burns up.

Gardian has many good points....If one wants to understand the ramifications of increased atmospheres, one needs to understand gas. For example, the Ideal Gas law PV=nRT with P=Pressure, V=Volume, n=number of moles, R=Universal gas constant and T=Temperature. In looking at this equation, when one side increases the other side will need to as well. R is a constant so the amount of gas and or the temperature will change. If we were to double the atmospheric pressre and not change the volume it occupies there would need to be an external force to compress the gases in the atmosphere which would indead increase the temperature assuming the amount of gas is the same as it is now. This would not result in a dry earth but would change the climate. Clouds and rain are formed by water condensing but under increased pressure, it would be difficult for clouds to form especially with increased pressure. If they did form, they would be low to the ground like a fog. There are other sinarios one can play with but to answer the question posted here we go... What I was able to uncover and from my studies in chemistry and physics is that under 2 atmospheres dissolved gases are significantly greater. This would effect all life on earth from bottom dwellers in oceans and lakes to the mountain tops. Gases are considered Nutrients and would be more available so the sizes of all life would be greater especially those limited by O2 absorbtion like shelled or exoskeleton creatures. All liquids and soft tissue would contain more gases which effect rates of reactions and cell functions. This is why athletes use hyperberic chambers because it speeds up the rates related to repairing functions in the body. I read up on the Japanese Scientist who did the work with the tomato plant, it's true but he also played with different wavelengths of light and varied the intensities...he removed all the harmful UV wavelengths and just used concentrated visible light. Pretty cool what he was doing...it grew 40ft tall and in a 3 mounth period the tomato plant yielded 3,000 ripe tomatoes!!! Crazy! Dr. Tao?... So in short, my answer to the question is things would be bigger and stronger physically and with better nutrition and regeneration, would probably live longer. How long is unsure. This would also affect all living species. I know that genetics plays the biggest role in the phenotipical characteristics of a species (like size) but I do find it interesting that in the fossil record there are quite a few large species that are similar to species we have in todays world that are much smaller.....makes you wonder....good question!

ouch!

I saw on youtube a video, where a scientist did some amazing research.. He grew a species of piranhas in a tank of doubled atm pressure.. And the result he got: the piranhas were double the size in comparison to the original ones. I study my first year of chemistry in university and it just hit me the other day, that it would be awesome to do something similar to that. Youknow.. experiments on increased pressure in atmosphere and it's effects on living organisms or even animals.. I thought about testing this on some ants, or sumthing similar. Call me crazy but I think it would be awesome to try this out.. Have anyone of you got to do something related to this kind of study?