Know It All: Friday’s top stories at NBC News

Glaciers in Antartica Rodrigo Arangua / AFP – Getty Images file

Here are some of the top stories we’re following at NBC News:

Top scientists: Man-made global warming ‘extremely likely’

The Intergovernmental Panel on Climate Change says it is “extremely likely” that human beings are driving global warming in a new report – some of the strongest words scientists have used to date to describe the effect mankind if having on the changing climate. The seas are getting warmer, snow and ice are melting, and greenhouse gases are up, scientists said. Read more at NBC News.

Investigation ordered into Washington Navy Yard shooting

Navy Secretary Ray Mabus has ordered an investigation into the shooting at the Washington Navy Yard that left 12 people dead and four others wounded, as the remains of gunman Aaron Alexis were released by the D.C. medical examiner. A memo ordering the probe sought details on the events leading up to the shooting and an investigation into the shooter’s mental health and military background, according to the Associated Press. Read more at NBC News.

Official: No clear evidence American involved in Kenya attack

Despite claims by Somalia-based militant group Al Shabaab that Americans were involved in a deadly rampage at a Kenyan mall, there is no clear evidence linking any American individuals to the attack, a U.S. intelligence official said. A number of names provided by the terrorist group, which has links to al Qaeda, have not shown any connection to a real person, according to the official. Dozens of U.S. investigators have arrived in Kenya in recent days. Read more at NBC News.

DNA pioneer’s genetic prescription: Have kids early

James Watson, who won a Nobel Prize for his work in discovering the double-helix structure of DNA, has a prescription for parents who want to avoid passing on bad genes: Start having kids early. The odds of passing on the traits for mental illness is about five percent – but those odds rise the longer wannabe moms and dads wait to get busy, Watson says: “And it’s probably true for men and women as well.” Read more at NBC News Science.

Experts: Obamacare glitches? Don’t sweat it.

Republican lawmakers giddy about the prospect of hiccups in the rollout of health insurance exchanges – one of the main components of health care reform – shouldn’t get their hopes up, experts said. Small glitches are to be expected and won’t have much of an impact on the overall system, said Jay Angoff, a health insurance expert who helped set up the exchanges at the Department of Health and Human Services. The plans won’t go into effect until Jan. 1, 2014, according to HHS Secretary Kathleen Sebelius. Read more at NBC News Health. And Dr. Nancy Snyderman answers some of the most common questions about the Affordable Care Act.

Report: US inspects for WMDs at only half of ‘high-risk’ seaports

Amid budget cuts that have whittled away its workforce, the number of U.S. inspectors at foreign ports is down, and nearly half the seaports considered high risk for shipping weapons of mass destruction to the U.S. go without inspection teams. An upcoming Government Accountability Office report obtained by NBC News concludes that while “there have been no known incidents of cargo containers being used to transport WMD, the maritime supply chain remains vulnerable to attacks.” Read more at NBC News Investigations.

Two weeks after baby? More new moms cut maternity leave short

More new mothers are feeling professional and financial pressure to cut their maternity leave short, sometimes giving themselves only a few weeks or sometimes days after giving birth to return to the job. Around two-thirds of American moms are employed while pregnant, according to the National Center for Health Statistics. But while the average maternity leave is 10 weeks, about half of new moms only give themselves five. Read more at TODAY.