Ethics in the twentieth century had some unique characteristics. First, the prevailing philosophical notions of uncertainty and relativism served to undermine ethics with an underlying belief that all ethics is relative. With this assumption, it is hard to justify the pursuit of any moral understanding because there would ultimately be no bedrock. Hence, ethics, in the 20th century was not viewed as particularly relevant. Second, many puzzling ethical problems were extreme endgame problems. People were concerned about blowing up the planet, destroying the ozone layer, destroying the rainforests, or getting clobbered by an asteroid. These were not subtle ethical issues. Clearly we wanted to avoid destroying ourselves or avoid being destroyed. This led to a black and white view of ethics. When more subtle issues were confronted, such as euthanasia or abortion, people applied the same black and white reasoning leading to polarization and little progress.
The third characteristic of ethics in the twentieth century, was this simplistic black and white view of ethical reality. We cared deeply about endgame scenarios and virtually ignored other concerns as irrelevant. Science fiction literature, from the middle of the century on, posed a variety of moral problems from time travel, to colonizing other planets that sparked the imaginations of many but received very little serious discussion because of the unlikelihood that we would ever confront them. Since the concerns of ethics in the twentieth century were either extreme endgame scenarios that we worried excessively over (such as nuclear annihilation) or preposterous fantasy scenarios that we cared little about (such as going back in time and killing your grandparents) most people viewed ethics as only slightly relevant to daily life.
In the 21st century, the advances in the biological sciences will yield a different perspective. Greater understanding of the brain will lead to the conclusion that our perceptions of reality are based in our biological make up. This will 'cure' the relativism of the 20th century because, although nothing is absolution and nothing is certain, there are a limited range of possibilities to be considered. Objective reality does not exist outside us, it exists inside of us. That is to say we are likely to have the same consistent impressions of the world because we share the same biology. This will lead to a form of biological realism that will anchor the excessive relativism of the 20th century. As we learn more about intentional states and brain chemistry, we will learn more about epistemology. We will also learn more about the foundations of ethics. Hume's concept of an inborn moral sympathy is likely to be revisited as moral intentional states.
A second difference in ethics that will manifest itself in the 21st century is in the moderation of brinkmanship. In the 20th century advances in science and technology were often viewed as tinkering with nature because the potential price of progress was often so high. In the 21st century we will see progress as more incremental. Diseases will be cured. Life extended. Quality of life improved. Instead of the all or nothing at all scenarios of time travel, extraterrestrials and nuclear annihilation we will see progress as something that we have a say in. We will decide, through public debate, how we feel about clones, spare parts, euthanasia and a wide variety of as yet uninvented biotechnologies.
The third difference that we will see in ethics is that we will no longer view ethics as black and white. We will see many shades of gray in our ethical decisions, which will encourage more people to involve themselves in the discussions. Instead of questions such as - Do we want to annihilate ourselves? (which is not really a worthwhile question) - we will address questions such as How long should life be extended and at what cost ? Our decisions on these issues will not be final decisions. We will come back and revisit them as often as we need to in order to resolve them satisfactorily.
Moral philosophy will move to the foreground in the 21st century. Epistemology asks the question - What is ? ; while moral philosophy asks the questions - What should be ? Greater understanding in the biological sciences will leave us less restricted by "What is" and allow us to pursue "What should be." Developing methods for determining what should be may be one of the most difficult problems in philosophy in the 21st century. This shift from epistemology to moral philosophy in the 21st century may be as dramatic as the shift from scholasticism to secularism in the 16th century. Secularism turned away from religious texts as the sole source of truth and began to look at the 'real' world. This, of course, led eventually to the rise of science and the dominance of scientific realism in the early twentieth century. It is quite likely that the end of the 20th century will mark the end of the age where we look to physical science for truth and begin to look at more speculative epistemologies.