The paragraph near the end addresses this, somewhat (to keep us from feeling too good):

"The fact that things have in fact been getting better – overwhelmingly so – does not guarantee progress in the future. After years of easy money and debt financing of companies and governments, a large-scale financial crisis is possible, when all the bills come due. Global warming may threaten ecosystems and affect the lives of millions. Large-scale war between major powers is possible. Terrorists could wreak havoc on a massive scale ..."