If we do happen to somehow achieve immortality before the end of the century, the glory of it all won't last long. The coming ecological catastrophe that is global climate change will make sure most people won't be sticking around more than they should be. The level of change and cooperation needed to avoid it is impossible if somehow we decide that we deserve to be on earth longer than was ever designed.

This is a classic dystopian futuristic narrative, and it has a very simple logical structure: if x, y, and z technologies allow us to do something, and all of our current problems remain the same, then catastrophe will ensue. The same type of reasoning was used to suggest that there would be a globally catastrophic famine before 2000 back in 1900 because many demographers predicted that the Earth would have more than 6 billion humans (when are people going to realize Thomas Malthus was wrong!). They reasoned there would be a global famine because they understood that population in the 20th century was going to increase dramatically (which it did) but did not factor in how our technologies would also increase food production (which they did) = no global catastrophic famine. The logic of this argument is the same. Right now climate change is a major issue, and we need to address it. The important thing to remember is that we will. Renewable energy technologies are not going away... they are only getting better, and they will eventually become our main source of energy (despite how much the fossil fuel industry fights it). Also, there are other technologies that will be developed this century (e.g., nuclear fusion) that will make energy super abundant and cheap for the entire planet - with no negative side effects. This is not far fetched - this is following current trends (EXAMPLE of trend: The average person in Britian in 1800 had to work 6 hours for 1 hour of candle light. The average person in Britain today has to work half a second for 1 hour of electric light (Ridley, 2010).). Also, there are already nanotechnologies being developed that will be able to modify the chemical composition of the atmosphere. It is extremely likely that by the 2030s will be be able to use either genetically engineered bacteria or nanotechnology (or both) to really balance out our atmospheric composition to desired levels.

Let me be clear, there will be problems in the future. There may be bigger problems in the future than the problems we are facing now. BUT the problems we are facing now won't be problems in the future, because we are a very adaptable species. When we recognize a problem we dedicate a tremendous amount of resources to solving the problem. We don't solve problems perfectly, and never will, but adaptation is strong and the problem will, given enough time, be overcome. Also, the second issue krisc brought up was about overpopulation. This isn't true either. The demographic transition, which is a process that has been happening for centuries and will be completed this century, will LOWER our overall population by 2100. Most estimates, including by the UN, claim that our population will top out at a maximum of 10 billion people before starting to decline. I'm not going to go into detail about the demographic transition, but if you haven't read about it yet, do so. The logical conclusion of the demographic transition is that people will live indefinitely but have no offspring.

Finally, overpopulation won't be as big of an issue because you are supposing that we will have more people with the same level of access to resources and energy. This won't be the case (as I argued above). When we transition to a Type 1 civilization we will have complete control of all resources on our planet (with no negative side effects) and we could essentially host far more people on our planet than we do now (even though we may not need to). Of course, we may have more overall intelligent beings on the planet in 2100 since most (or all) will likely be non-biological, and I can't make predictions about post-singularity populations.